Source: http://www.sumobrain.com/patents/wipo/Lamp-cover-flash-unit/WO2012060777.html
Timestamp: 2013-05-20 17:45:10
Document Index: 465944744

Matched Legal Cases: ['art 372', 'art 372', 'art 372', 'art 372', 'art 372', 'art 372', 'art 374']

LAMP COVER FOR A FLASH UNIT - XENON TECHNOLOGIES PTE. LTD.
LAMP COVER FOR A FLASH UNIT
WIPO Patent Application WO/2012/060777 Kind Code:
The present application provides a lamp cover (202, 278, 346, 358, 386, 394) for closing a window (43) of a flash unit (200, 276). The lamp cover (202, 278, 346, 358, 386, 394) comprises a first array of optical zones (254, 330, 347, 361, 388, 390, 392, 396) that are contiguously connected to each other on a surface (63) of the lamp cover (202, 278, 346, 358, 386, 394). One or more of the optical zones (202, 278, 346, 358, 386, 394) have a pitch (255, 342, 362) of 0.07 millimeter at most for visually blurring objects (34, 36, 38, 44, 48, 54, 56) placed behind the lamp cover (202, 278, 346, 358, 386, 394). The application also provides a method for making or using the flash unit (200, 276), which comprises a step of sealing the window (43) with the lamp cover (202, 278, 346, 358, 386, 394). The application further provides a method for making the lamp cover (202, 278, 346, 358, 386, 394), which comprises a step of injecting a molten optical plastic material into a mould insert (262) for providing optical zones (254, 330, 347, 361, 388, 390, 392, 396) or Fresnel zones (254, 330, 347, 361, 388, 390, 392, 396) and a step of detaching the lamp cover (202, 278, 346, 358, 386, 394) from the mould insert (262). Inventors:
ZHOU, Jianghong (47 Ayer Rajah Crescent, #06-05, Singapore 7, 13994, SG) Application Number:
SG2011/000384 Publication Date:
XENON TECHNOLOGIES PTE. LTD. (47 Ayer Rajah Crescent, #06-05, Singapore 7, 13994, SG) ZHOU, Jianghong (47 Ayer Rajah Crescent, #06-05, Singapore 7, 13994, SG) International Classes:
F21V5/04; F21V3/02; G03B15/02 Attorney, Agent or Firm:
KANG, Alban et al. (ATMD Bird & Bird LLP, 2 Shenton Way #18-01SGX Centre 1, Singapore 4, 06880, SG) Download PDF:
CLAIMS 1 . A lamp cover (202, 278, 346, 358, 386, 394) for a flash unit (200, 276), the lamp cover formed from a mainly transparent material, comprising: a first array of optical zones (254, 330, 347, 361 , 388, 390, 392, 396) on a surface (63) of the lamp cover (386, 394), the optical zones (254, 330, 347, 361 , 388, 390, 392, 396) having a pitch (255, 342, 352) smaller than 0.07 millimeter, the pitch establishing an optical effect that obscures a view of functional components (34, 36, 38, 44, 48, 54, 56) positioned behind the lamp cover (202, 278, 346, 358, 386, 394). 2. The lamp cover (202, 278, 346, 358, 386, 394) of claim 1 , wherein the optical effect includes transforming an appearance of the mainly transparent material from a transparent appearance to an opaque appearance. 3. The lamp cover (202, 278, 346, 358, 386, 394) of claim 1 or claim 2, wherein at least one of the optical zones (254, 330, 347, 361 , 388, 390, 392, 396) includes a Fresnel zone (254, 330, 361 , 388, 390, 392, 396) having an equivalent profile (258, 334, 356, 364) is aspheric. 4. The lamp cover (202, 278, 346, 358, 386, 394) of claim 3, wherein the equivalent profile (258, 334, 356, 364) is defined by a sag equation in the form of wherein an optic axis (46) of the equivalent profile (258, 334, 356, 364) lies in a Z-axis (46) direction such that z(r) includes a sag component of the equivalent profile (258, 334, 356, 364) from a vertex (1 53) at a distance r (1 38, 1 52) from the Z-axis (46); and coefficients a, (t = 1, 2, 3, ...as natural numbers) includes an expansion of the equivalent profile (258, 334, 356, 364) from an axially symmetric quadric aspheric equivalent profile (258, 334, 356, 364) specified by constants R and κ. 5. The lamp cover (202, 278, 346, 358, 386, 394) of claim 3 or claim 4, wherein the equivalent profile (258, 334, 356, 364) includes a convex portion (336, 368, 370). 6. The lamp cover (202, 278, 346, 358, 386, 394) of any of the claims 3 to 5, wherein the equivalent profile (258, 334, 356, 364) includes a concave portion (338, 340, 366). 7. The lamp cover (202, 278, 346, 358, 386, 394) of claim 6, wherein the concave portion (338, 340, 366) is near a central area (259) of Fresnel zones (254, 330, 347, 361 , 388, 390, 392, 396) and the convex portion (336, 368, 370) is near an end area (341 , 343) of Fresnel zones (254, 330, 347, 361 , 388, 390, 392, 396). 8. The lamp cover (202, 278, 346, 358, 386, 394) of claim 6, wherein the convex portion (336, 368, 370) is positioned near a central area (259) of Fresnel zones (254, 330, 347, 361 , 388, 390, 392, 396) and the concave portion (338, 340, 366) is positioned near an end area (341 , 343) of Fresnel zones (254, 330, 347, 361 , 388, 390, 392, 396). 9. The lamp cover (202, 278, 346, 358, 386, 394) of any of the claims 3 to 8, wherein some of the Fresnel zones (254, 330, 347, 361 , 388, 390, 392, 396) has a pitch (255, 342, 362) of 0.04 millimeter at most. 10. The lamp cover (202, 278, 346, 358, 386, 394) of claim 9, wherein the some of the Fresnel zones (254, 330, 347, 361 , 388, 390, 392, 396) have a pitch (362) of 0.01 millimeter at most. 11. The lamp cover (202, 278, 346, 358, 386, 394) of any of the claims 3 to 10, wherein an equivalent radius (r) (138, 152) of the equivalent profile (254, 330, 347, 361 , 388, 390, 392, 396) is less than 5 millimeter. 12. The lamp cover (202, 278, 346, 358, 386, 394) of any of the claims 3 to 11 , wherein at least some of the Fresnel zones (254, 330, 347, 361 , 388, 390, 392, 396) include a linear Fresnel zone (254, 330, 347, 361 , 390, 392). 13. The lamp cover (202, 278, 346, 358, 386, 394) of any of the claims 3 to 12, wherein at least some of the Fresnel zones (254, 330, 347, 361 , 388, 390, 392, 396) include a curved Fresnel zone (388, 396). 14. The lamp cover (202, 278, 346, 358, 386, 394) of claim 13, wherein the curved Fresnel zone (388, 396) includes a circular Fresnel zone (388, 396). 15. The lamp cover (202, 278, 346, 358, 386, 394) of claim 1 or claim 2, wherein some of the optical zones (254, 330, 347, 361 , 388, 390, 392, 396) are provided on an internal surface (63) of the lamp cover (386, 394). 16. The lamp cover (202, 278, 346, 358, 386, 394) of any of the claims 1 , 2 and 16, further comprising a second array of optical zones (254, 330, 347, 361 , 388, 390, 392, 396). 17. The lamp cover (202, 278, 346, 358, 386, 394) of claim 16, wherein the first array of optical zones (254, 330, 347, 361 , 388, 390, 392, 396) and the second array of optical zones (254, 330, 347, 361 , 388, 390, 392, 396) are provided on opposite surfaces (63, 64) of the lamp cover (202, 278, 346, 358, 386, 394) respectively. 18. The lamp cover (202, 278, 346, 358, 386, 394) of claim 1 , wherein mainly transparent material includes an optical plastic material for injection molding. 19. Flash unit (200, 276) having a flash lamp (34) for generating artificial flash light, a casing (42) enclosing the flash lamp (34), the casing (42) further comprising a window (43) for emitting the artificial light, and the lamp cover (202, 278, 346, 358, 386, 394) according to any of the preceding claims for fitting onto the window (43). 20. Portable electronic device having the flash unit (200, 276) of claim 18. 21. Method for making the flash unit (200, 276) of claim 19 comprising sealing a window (43) of a casing (42) with the lamp cover (202, 278, 346, 358, 386, 394) according to any of the preceding claims. 22. Method for making the lamp cover (202, 278, 346, 358, 386, 394) according to any of the claims 1 to 18 comprising injecting a molten optical plastic material into a mould insert (262) for providing Fresnel zones (254, 330, 347, 361 , 388, 390, 392, 396) of the lamp cover (202, 278, 346, 358, 386, 394), and detaching the lamp cover (202, 278, 346, 358, 386, 394) from the mould insert (262).
LAMP COVER FOR A FLASH UNIT [0001] The present application relates to a lamp cover for a flash unit and a method for making the lamp cover. More specifically, the present application relates to a lamp cover with optical zones including Fresnel zones. [0002] The flash unit is often used in photography for producing a burst of artificial light to illuminate a dark scene, typically from 0.0001 to 0.005 second. The flash unit, also known as a flash or an electronic flash unit, includes a discharge lamp that is enclosed by a casing. Typically, one or more reflectors are installed inside the casing for directing the light from the discharge lamp towards an opening on the casing. The opening is sealed by the lamp cover for transmitting the light to the dark scene and for shielding internal components of the flash unit. [0003] In recent years, consumers become increasingly interested in purchasing a mobile phone handset with an in-built flash unit for photography. The in-built flash unit has to be sufficiently small for being integrated into the handset, whilst the handset itself is usually compact. However, the small in-built flash unit is still required to provide substantially uniform illumination over a large area at a distance for satisfactory photographing. Hence, manufacturers of the flash unit face daunting technical challenges in providing a miniaturized flash unit that is capable of producing adequate illumination for photo taking. Since the mobile phone sets are usually designed to be visually appealing, it would be desirable for the manufacturers to conceal internal structures of the in-built flash units, because these internal structures may diminish attractive appearance of the mobile phone sets. [0004] The present application seeks to present a lamp cover which offers good shielding, provides smart appearance, and facilitates satisfactory illumination for a miniaturized flash unit. [0005] According to a first aspect, a lamp cover is provided for closing a window of a flash unit. The lamp cover can be substantially transparent, translucent or semitransparent such that the lamp cover is capable of transmitting with low absorbance value or small absorption factor. The lamp cover may be impregnated with certain color(s) or has specific type(s)/mixture(s) of optical material(s). Alternative names of the lamp cover can include lamp shield, screen, cover, lens, plate or the like. The window is an opening or orifice of the flash unit, which can be a part or an entire side of the flash unit. The window may also be on a casing or at a corner of flash unit such that the window occupies two or more sides of the flash unit. The flash unit can include a flashbulb, a flashcube, an electronic flashtube or a LED flash. [0006] The lamp cover includes a first array of optical zones that are connected to each other. The optical zones are discrete optical regions which can be spaced part from each other or contiguously connected to each other. For example, the optical zones can be linearly or serially connected to each other. The optical zones can also enclose each other, such as in the form of concentric rings. Alternatively, the optical zones can be neighbouring each other in tessellation or tiling forms, such as in a honeycomb pattern. The optical zones can be located on a single side or multiple sides/surfaces of the lamp cover. The optical zones can alternatively be discontinuous or detached from each other. [0007] One or more of the optical zones have a size of 0.07 millimeter (mm) at most for visually blocking or blurring objects behind the one or more optical zones. The size describes a distance between boundaries of the optical zone. If the optical zones are linearly distributed and are contiguous to each other, the distance between two optical zones that are both contiguous to an optical zone joining the two optical zones can be termed as a pitch of the optical zones. In this case, the pitch can also represent a breadth of the middle optical zone. The pitch is alternatively termed a size or a dimension for delimiting an optical zone. The optical zones describe regions of the lamp cover with similar geometries or shapes for achieving desired optical effects. [0008] The optical zone is limited to have a pitch of 0.07 millimeter for preventing an observer in front of the lamp cover from seeing through the lamp cover clearly. Since a naked human eye typically has difficulty in differentiating an object of 0.07 millimeter or less at the least distance of distinct vision, usually about 250 millimeter, a lamp cover whose surface is fully covered by discrete optical zones of less than 0.07 millimeter pitch may appear to be opaque or reflecting. Thus, although the lamp cover itself can be built by a transparent material, the naked eye will have difficulty in viewing parts or structures behind the lamp cover. This optical effect is useful for manufacturers of the flash unit because internal components of the flash unit may become blurred or even invisible to users of the flash unit who can only view the flash unit externally. The internal components and their wire connections are prevented from impairing aesthetic appearance of the flash unit. A shining, reflective or uniform outlook of the lamp cover will help to enhance an image of a mobile phone that is installed with flash unit. [0009] The optical zones can have varying sizes/dimensions, shapes, pitches or distribution patterns for covering a portion, an entire side or several sides of the lamp cover. Each of the optical zones can have different shape, curvature, size or color. For example, groups of the optical zones can form any regular (e.g. honeycomb pattern) or irregular formations (e.g. randomly scattered) on one of more parts of the lamp cover. [0010] The optical zones can include Fresnel zones whose equivalent profile may conform to an aspheric surface. In other words, the Fresnel zones form a Fresnel lens that can refract an incident light for photography. The Fresnel lens has a short focal length without the mass and volume of material that would be required by a typical or standard lens, such as a biconvex lens, a plano-convex lens, a positive/negative meniscus lens, a plano-concave lens and a biconcave lens. Compared to these typical lenses, the Fresnel lens is much thinner and flatter for capturing more oblique light from a light source, thus allowing uniform illumination over much greater distances. The Fresnel lens is especially useful for providing a miniaturized flash unit that provides satisfactory illumination. [0011] In each of these Fresnel zones, an overall thickness of the typical lens is decreased, effectively dividing the continuous surface of the typical lens into a set of surfaces of the same curvature, with stepwise discontinuities between them. In fact, the Fresnel lens can have an array of prisms arranged in a circular fashion, with steeper prisms on the edges and a nearly flat convex region at center. If these prisms have substantially uniform sizes and are closely packed against each other contiguously in their width direction, the width is also known as the pitch of the Fresnel zones. The term width may alternatively be known as length or height, depending on orientations. [0012] The equivalent profile of one or more of the Fresnel zones can be derived from a spherical lens by dividing the spherical lens into a set of concentric annular sections. The concentric annular sections can also be in the form of elongated stripes, known as linear Fresnel zones. The Fresnel zones have an appearance of an uneven surface such that the Fresnel zones are also called teeth or steps of a Fresnel lens. The profile of the spherical lens is called an equivalent profile or a spherical surface. A radius of the spherical lens is accordingly called equivalent radius. In some situations, the equivalent profile may conform to a surface of an aspheric lens or an aspheric surface. The aspheric lens is a lens whose surface is curved, but is not a portion of a sphere or cylinder. An equivalent profile of the aspheric lens may alternatively known as an aspheric surface. The Fresnel zones that form a Fresnel lens can be integrated into the lamp cover. In other words, the lamp cover can incorporate the Fresnel lens, which may be aspheric. [0013] The equivalent profile or the aspheric surface may follow a sag equation, which is z(r) [0014] In particular, an optic axis of the equivalent profile lies in a Z-axis direction such that z(r) represents a sag component of the aspheric surface from a vertex at a distance r from the Z -axis. The coefficients ; ( t = 1, 2,3 ... as a natural number) represent a deviation of the aspheric surface from an axially symmetric quadric aspheric surface, which is specified by constants R and κ. The sag equation provides a convenient way for automated manufacturing techniques, such as by injection moulding, grinding or polishing. The sag equation also enables simulation software packages to predict optical performance of an aspheric lens such that a designer can adjust various parameters of the sag equation for achieving desired results. For example, the sag equation facilitates to achieve desirable optical performance including guide numbers over a flash coverage area for effective refraction of the flash light. A guide number represents an exposure constant for a flash unit that measures the ability of the flash light for illuminating a subject to be photographed at a specific film or sensor sensitivity and angle of view. Typically, a higher guide number indicates a more powerful flash. The guide number is related to the product of a distance from the flash unit to an object to be illuminated and root square of the time integral of luminance at the center of the object to be illuminated. [0015] The sag equation may appear in an odd form or an even form, although an even form is more frequently used in industry. An odd form of sag equation is shown [0016] An even form of the sag equation is represented as [0017] In the above-listed equations, both ?. and are coefficients (similar to ) for representing a deviation of the aspheric surface from an axially symmetric quadric aspheric surface, which is specified by constants R and κ. . [0018] The equivalent profile may include a convex portion. The convex portion can focus incident light towards a targeted area such that the light becomes convergent. Hence, the convex portion is useful for concentrating the incident light towards a narrow region for increasing its brightness. [0019] Alternatively, the equivalent profile can include a concave portion. The concave portion can expand the incoming light towards a broad area such that the light becomes more divergent. Therefore, the concave portion is useful for spreading the light in order to illuminate a wide area. [0020] Both the concave and convex portions are labeled according to an orientation of the optical axis z(r) . Although curvatures of lenses are usually determined by air/glass interfaces, the optical axis z(r) is adopted here for labeling for convenience. If a curvature of a lens is convergent, the lens is convexly shaped. If a curvature of a lens is divergent, the lens is concavely shaped. Using the optical axis z(r) , a curvature that protrudes in a positive direction of the optical axis z(r) is considered as the convex portion, whilst another curvature that bulges in a negative direction of the optical axis z(r) is termed as the concave portion. When describing the lamp cover, a positive direction of the optical axis z(r) is often chosen to be from an interior of the flash unit (e.g. flash lamp) towards an exterior of the flash unit (e.g. towards the lamp cover). [0021] The concave portion can be near a centre of the Fresnel zones and the convex portion can be near one end of the Fresnel zones. Alternatively, there are two convex portions near two opposite ends of the Fresnel zones respectively. The opposite ends are located in a lateral direction of the lamp cover, whilst the optical axis the optical axis z(r) lies in a longitudinal direction of the lamp cover (e.g. thickness direction). The central concave portion can cause divergence of the flash light from a centre of a flash tube to cover a broad area on an object. The convex portion near the edge can bring otherwise escaping light back to the object for increasing its brightness. [0022] In contrast, the convex portion may be near a centre of the Fresnel zones and the concave portion may be near an end of the Fresnel zones. For example, the convex portion can be positioned at a middle location of a linearly distributed Fresnel zones. Moreover, there can be two concave portions near two opposite ends of the Fresnel zones if the Fresnel zones are linearly distributed. Positions and gradients of the concave and/or convex portions can affect the guide number of the flash unit using the lamp cover. To increase the guide number without compromising illumination distribution, the two concave portions at opposite ends of the lamp cover can be brought further away from the central convex portion. Alternatively, increasing the gradient of the central convex portion or/and decreasing the concave portions can also increase the guide number while maintaining the illumination distribution. Larger size or greater gradient of the concave portion(s) help to illuminate center of light coverage area. Lesser or more gentle gradient of the convex portion also assist in brightening edges of light coverage. [0023] One or more of the Fresnel zones can have a pitch of 0.04 millimeter or less. Using 0.04mm as the pitch enables much larger curvature of the Fresnel lens so that it will have more convergent effect to achieve higher guide number. [0024] The aspherical Fresnel zones with a convex portion, a concave portion or any combination of these can join effects of a positive focal length Fresnel lens used as a collector, effects of a positive focal length Fresnel lens used as a collimator, and effects of a negative focal length Fresnel lens used as a divergener. Hence, the Fresnel lens with the aspherical profile is very flexible in providing various desired optical effects for illumination. [0025] One or more of the Fresnel zones can have a pitch of 0.01 millimeter or less. Since a diamond cutter can cut a feature in a mould or a mould insert up to 0.001 millimeter, the Fresnel zones with the pitch of from 0.01 to 0.001 millimeter or less can be precisely made with satisfactory surface finish quality. Accordingly, the Fresnel lens with the pitch of 0.01-0.001 millimeter allows more flexibility in design for various optical effects. It also reduces light loss from draft facets because of smaller Fresnel teeth height of each Fresnel zone. [0026] An equivalent radius (r) of the equivalent profile may be less than 5 millimeters. The small equivalent radius enables the Fresnel zones to refract an incident light more effectively by achieving a larger refractive angle as compared to that of a larger radius. The Fresnel lens zones can thus be designed to follow a more complex aspheric profile for special optical effects, such as localized light divergence of convergence. The lamp cover with the Fresnel zones or Fresnel lens can be suitable for both close-range photography (i.e. up to a distance of 50 centimeter) and mid- range photography (i.e. up to a distance 3 meter). [0027] The Fresnel zones can include linear Fresnel zones. The linear pattern is viewed from a top of the Fresnel zones, whilst a cross sectional view of the Fresnel zones presents their teeth pattern or pitches. The linear Fresnel zones include elongated strips whose edges are substantially straight. The edges of the elongated strips may be parallel to each other and these Fresnel zones may be contiguously connected to each other, although their joints may have sharp edges or ridges for dividing them. A dimension of one of the Fresnel zones in the width direction of the Fresnel zones can be termed as the pitch, which defines a size of a Fresnel zone, direction of the width is perpendicular to the optical axis z(r). In a length direction of the Fresnel zone, which is perpendicular to the width direction and the optical axis z(r) there is no limitation to its size. When the Fresnel zones are closely packed together, the pitch also provides a distance between two Fresnel zones that are located at opposite sides of a Fresnel zone in-between. The Fresnel zones may follow a spherical or an aspherical equivalent profile. [0028] The Fresnel zones can include curved Fresnel zones. The curved pattern is viewed from a top of the Fresnel zones, whilst a cross sectional view of the Fresnel zones still presents their teeth pattern or pitches. Any of the curved Fresnel zones can follow a portion of a curve, such as a portion of an asteroid, a bean curve, a bicorn, a bicuspid curve, a cardioid, a conic section, a parabola, a hyperbola, an ellipse, a spiral, a lituus, a clothoid or a cycloid. The Fresnel zones may reside on an uneven surface that follows an aspheric profile or any other uneven base. The Fresnel zones, including both the linear and the circular Fresnel zones, may have regular patterns of Fresnel zones with uniform, incremental, decremental or other varying pitches. [0029] In an alternative, the curved Fresnel zones can include circular Fresnel zones. The circular pattern is viewed from a top of the Fresnel zones, whilst a cross sectional view of the Fresnel zones again presents their teeth pattern or pitches. One layout of circular Fresnel zones includes concentric rings that are next to each other. A breadth of any of the circular Fresnel zones can be termed as the pitch of the Fresnel zones. The concentric Fresnel zones may have oval, square, rectangular, triangular or any other regular shapes. [0030] The lamp cover can further include another or a second array of Fresnel zones that has a similar or dissimilar pattern to the first array of Fresnel zones. The two arrays of Fresnel zones can be next to each other, enclosing each other or detached from each other. For example, the first array of Fresnel zones in a circular form encloses the second array of Fresnel zones in a linear form. The first array of Fresnel zones can be symmetrical to the second array of Fresnel lens with respect to a centerline of the lamp cover. [0031] In an embodiment, the optical zones or Fresnel zones are provided on an internal side or an interior surface of the lamp cover. The internal side is hidden and not accessible during normal use of the flash unit. When assembled, the lamp cover typically seals an opening/window of the casing, which is a container of the flash unit. The concealed internal side, in contrast to the exposed external side of the lamp cover, is not contactable from an external of the flash unit. Thus, optical zones of the lamp cover will not be exposed for contamination or tarnishing by external rubbing or dirt accumulation. Consequently, optical effectiveness of the lamp cover will not be affected by the usage, which subjects the lamp cover to rubbing, knocking, or grease accumulation. [0032] In one embodiment, the first array of optical/Fresnel zones and the second array of optical/Fresnel zones are provided on two opposite sides of the lamp cover respectively. In this case, incident light that is refracted by the first array of optical/Fresnel zones is further refracted by the second array of optical/Fresnel zones, or vice versa. This arrangement doubles the effect of refraction by arranging the two arrays of optical/Fresnel zones on a single light path. Optical effects, including diverging, converging or localized illumination can be achieved more effectively by employing a single lamp cover. [0033] The lamp cover can include an optical material for injection molding. The optical material includes one or more transparent and semi-transparent materials for refracting light. Examples of the optical material include optical plastic materials and various glasses. The optical plastic materials include acrylic, vinyl, polycarbonate, polystyrene, polyolefin, cycloolefin polymer and methyl methacrylate. [0034] The present application provides a flash unit that includes a flash lamp for generating the artificial flashlight. The flash unit further includes a casing for protecting and supporting the flash lamp. The casing includes a window or an opening for emitting the artificial light. The flash unit also includes the lamp cover for fitting onto the window. [0035] The flash lamp is sometimes termed as a flashlight, a flashbulb, a flashcube, a magicube (X-Cube), a flipflash, an electronic flashtube or any other illumination device that is capable of illuminating with a short duration (e.g. less than a few seconds) for photography. The casing is often in the form of a shell or a box which encloses and protects other components of the flash unit. One form of the casing is a rectangular cuboid with thin walls of about 0.5 to 1.0 millimeter thickness. The window is an aperture for releasing the flashlight. Accordingly, the lamp cover has a comparable size for sealing the window such that the other components are kept away from external access after installing the lamp cover onto the casing. Alternatively, the lamp cover with the Fresnel lens is molded together with the casing. When most pitches of the optical zones or the Fresnel zones are 0.07 millimeter or less, internal components of the flash unit are not clearly visible to a naked eye outside the flash unit although the lamp cover can be made of a transparent material. Various design patterns may be further applied to the optical zones or Fresnel zones for achieving desirable optical effects or aesthetic appearances. [0036] The application can provide a portable electronic device that includes the flash unit. The portable electronic device includes a mobile telephone handset, a photo/video camera, an audio player such as a MP3 player, a gaming device, a portable scientific instrument, or accessory for the portable electronic device. The portable electronic device can produce good quality photograph or record high-quality video with the illumination of the flash unit. Often, the portable electronic device has other functions, in addition to its photography function. The flash unit will greatly improve performance of the portable electronic device for photo-taking, especially by adopting the lamp cover on the flash unit. [0037] According to a second aspect, the application provides a method for making or using the flash unit. The method comprises a step of sealing a window of the flash unit with the lamp cover. Thus, the lamp cover acts as a shield for protecting the internal components of the flash unit. The lamp cover also transmits or refracts flash light that is generated from an interior of the flash unit. Dust is prevented from entering the flash unit. In some situations, the lamp cover is sealed against the flash unit with waterproof glue or rubber gasket such that the entire flash unit can function normally in a rain or under water. The lamp cover can also be hermetically sealed against the window by interference or force fitting. [0038] According to a third aspect, the application provides a method for making the lamp cover. The method comprises a step of injecting a molten optical plastic material into a mould insert for providing the optical zones or Fresnel zones of the lamp cover. The method further comprises a step of detaching the lamp cover from the mould insert. In other words, the lamp cover is produced by an injection molding technique. Compare to other techniques, including extraction, blow moulding, rotational moulding, thermoforming, compression moulding, transfer moulding, casting, cold forming, solid- phase forming which may still be useful for making the lamp cover, the injection moulding technique has low cost in manufacturing setup and is able to produce large quantity of such lamp covers with high precision. Especially, when a mould insert of the optical zones or Fresnel zones is made by a hard metal (e.g. stainless steel), such a mould insert can produce over 1 ,000,000 parts/shots with good precision and surface finishing. [0039] The accompanying figures illustrate embodiments and serve to explain principles of the disclosed embodiments. It is to be understood, however, that these drawings are presented for purposes of illustration only, and not for defining limits of the claimed inventions, where [0040] Figure 1 is a diagram illustrating some representative rays for indicating travelling paths of flash light. [0041] Figure 2 is a diagram illustrating an isometric view of a known Fresnel lens of a known flash unit for showing its Fresnel zones. [0042] Figure 3 is a diagram illustrating a first known lamp cover that incorporates a known Fresnel lens. [0043] Figure 4 is a diagram illustrating another known Fresnel lens. [0044] Figure 5 is a diagram illustrating another known lamp cover that incorporates the other known Fresnel lens. [0045] Figure 6 is a diagram illustrating a flash unit with some representative rays for indicating travelling paths of flash light in accordance with an embodiment. [0046] Figure 7 is a diagram illustrating an isometric view of a Fresnel lens, which is a part of the first flash unit. [0047] Figure 8 is a diagram illustrating a diamond cutter cutting a mould insert for making a Fresnel lens. [0048] Figure 9 is a diagram illustrating a flash unit with lines for indicating travelling paths of its flash light in accordance with another embodiment. [0049] Figure 10 is a diagram illustrating an isometric view of a second Fresnel lens, which is a part of a flash unit. [0050] Figure 11 is a diagram illustrating an isometric view of a Fresnel lens in accordance with another embodiment. [0051] Figure 12 is a diagram illustrating an isometric view of a Fresnel lens in accordance with another embodiment. [0052] Figure 13 is a diagram illustrating illuminance of flash coverage area versus flash coverage angle. [0053] Figure 14 is a diagram illustrating a first lamp cover with varying sizes of Fresnel zones. [0054] Figure 15 is a diagram illustrating a second lamp cover with densely allocated Fresnel zones. [0055] Exemplary, non-limiting embodiments will now be described with references to the above-mentioned figures. [0056] Figures 1-3 are diagrams illustrating a conventional flash unit 20. The first known flash unit 20 is also termed as a known flash unit 20 which comprises a flash lamp 34, two reflectors 36, 38 and a first known Fresnel lens 40 enclosed by a casing 42 (marked by the dotted line). The casing 42 has a window 43 that is covered by the first known Fresnel lens 40. More specifically, the flash unit 20 projects representative rays 22-32 for indicating travelling paths of its flash light. [0057] The flash lamp 34 is attached to a back reflector 44 on a backside of the casing 42. The Fresnel lens 40 is molded together with the casing 42 on a front side. The two reflectors 36, 38 are positioned between the Fresnel lens 40 and the flash lamp 34. The two reflectors 36, 38 are representative fir illustration, although there are more reflectors in the first lamp 34 in practice. The two reflectors 36, 38 consist of a left reflector 36 on a left side and a right reflector 38 on a right side of flash unit 20. The left reflector 36 is alternatively known as a first reflector 36 and the right reflector 38 is alternatively known as a second reflector 38. The back reflector 44, the left reflector 36 and the right reflector 38 are formed by a single piece of reflective material. An optical axis Z-axis 46 of the flash unit 20 is marked in a transverse direction from a centre (central position) of the flash lamp 34 towards the Fresnel lens 40. The Z-axis 46 is also known as an optical centerline of the flash unit 20. The six representative rays 22-32 are symmetrically plotted beside the Z-axis 46 in Figure 1 for showing light paths projecting from the flash lamp 34 towards the Fresnel lens 40. In fact, most components of the flash unit 20 and the rays 22-32 are symmetrically distributed with respect to the Z-axis 46 such that the Z-axis 46 also becomes a centerline 46 of the flash unit 20. [0058] The flash lamp 34 includes a glass tube 48 whose opposite ends 50, 52 are sealed by two electrodes 54, 56 respectively. A left electrode 54 (first electrode) is inserted into a left end 50 (first end) of the glass tube 48, whilst a right electrode 56 (second electrode) is inserted into a right end 52 (second end) of the glass tube 48. The two electrodes 54, 56 are soldered to a Printed Circuit Board or PCB (not shown) for controlling the flash unit 20. Xenon gas 60 completely fills an interior 58 of the glass tube 48 such that flash light can be emitted isotropically from the interior 58 when the flash lamp 34 is electrically charged. [0059] Both the left reflector 36 and the right reflector 38 are polished plates with mirror-like surface finish in silver color. The two reflectors 36, 38 face each other at an angle such that none of the two reflectors 36, 38 is parallel to the Z-axis 46. The left reflector 36 is located near the left end 50 of the flash lamp 34 and the right reflector 38 is positioned near the right end 52 of the flash lamp 34 respectively. [0060] The Fresnel lens 40 is situated further away from the two reflectors 36, 38 in a positive direction of Z-axis 46 towards the front. The Fresnel lens 40 includes multiple Fresnel zones 62 on its inner side 63 that faces the interior 58 of the flash unit 20. The Fresnel zones 62 have uneven surfaces in the form of teeth or steps on one side of a transparent material. An exterior side 64 of the Fresnel lens 40 is flat and smooth such that the exterior side 64 merges with an outline of the casing 42, which is substantially a cuboid. The exterior side 64 is accordingly known as an exposed side. The term cuboid is also known as a right cuboid, a rectangular box, a rectangular hexahedron, a right rectangular prism or a rectangular parallelepiped. The Fresnel lens 40 is made of acrylic material whose thickness direction is aligned to the Z-axis 46. [0061] The six rays 22-32 are emitted from the centre (marked as point "0") of the flash lamp 34 and pass through the Fresnel lens 40. A first ray 22, a second ray 24 and a third ray 26 are symmetrically disturbed against a fourth ray 28, a fifth ray 30 and a sixth ray 32 with respect to the optical centerline Z-axis 46. For simplicity, rays that land on or depart from the two reflectors 36, 38 are omitted. [0062] In particular, the first ray 22 is positioned to the left of the Z-axis 46 and composed of a first incident ray 66, a first refracted ray 68 and a first emitted ray 70. The three rays 66, 68, 70 are sequentially connected, originating from the centre point O. Refraction of the first incident ray 66, which occurs when passing through the glass tube 48, is omitted due to its little impact (e.g. refraction effect) without affecting an overall optical performance. According to Figure 1 , a first incident angle c<i is measured from the Z-axis 46 with a positive value. The first incident ray 66 is refracted when entering into the Fresnel lens 40 and becomes the first refracted ray 68. The first refracted ray 68 is further refracted when leaving the Fresnel lens 40, thus becomes the first emitted ray 70. A first emission angle βι of the first emitted ray 70, which is also measured from the Z-axis 46, is smaller than the first incident angle [0063] Similarly, the second ray 24 is located further to the left of the first ray 22 and composed of a second incident ray 72, a second refracted ray 74 and a second emitted ray 76. These rays 72, 74, 76 are sequentially connected, starting from the centre point O. As shown in Figure 1 , a second incident angle a 2 is measured from the Z-axis 46 with a positive value. The second incident ray 72 is refracted when entering into the Fresnel lens 40 and becomes the second refracted ray 74. The second refracted ray 74 is further refracted when leaving the Fresnel lens 40, thus becomes the second emitted ray 76. A second emission angle β 2 of the second emitted ray 76, which is also measured from the Z-axis 46, is smaller than the second incident angle a 2 . [0064] In a like manner, the third ray 26 is located further to the left of the second ray 24 and is composed of a third incident ray 78, a third refracted ray 80 and a third emitted ray 82. These rays 78, 80, 82 are sequentially connected, beginning from the centre point O. According to Figure 1 , a third incident angle a 3 is measured from the Z-axis 46 with a positive value. The third incident ray 78 is refracted when entering into the Fresnel lens 40 and becomes the third refracted ray 80. The third refracted ray 80 is further refracted when leaving the Fresnel lens 40, thus becomes the third emitted ray 82. A third emission angle β 3 of the third emitted ray 82, which is also measured from the Z-axis 46, is smaller than the third incident angle a 3 . [0065] The other three rays 28-32 are positioned at an opposite side of the Z-axis 46. In detail, the fourth ray 28 is located to the right of the Z-axis 46 and composed of a fourth incident ray 84, a fourth refracted ray 86 and a fourth emitted ray 88. The three rays 84, 86, 88 are sequentially connected, starting from the centre point O. Refraction of the fourth incident ray 84, which occurs when passing through the glass tube 48, is omitted due to its little impact without affecting overall picture. According to Figure 1 , a fourth incident angle a 4 is measured from the Z-axis 46 with a positive value. Here, the value of a 4 is equal to the value of ch. The fourth incident ray 84 is refracted when entering into the Fresnel lens 40 and becomes the fourth refracted ray 86. The fourth refracted ray 86 is further refracted when leaving the Fresnel lens 40, thus becomes the fourth emitted ray 88. A fourth emission angle β 4 of the fourth emitted ray 88, which is also measured from the Z-axis 46, is smaller than the fourth incident angle a . Here, the value of the fourth emission angle β 4 is equal to the value of βι. [0066] Similarly, the fifth ray 30 is located further to the right of the fourth ray 28 and composed of a fifth incident ray 90, a fifth refracted ray 92 and a fifth emitted ray 94. These rays 90, 92, 94 are sequentially connected, starting from the centre point O. As shown in Figure 1 , a fifth incident angle a 5 is measured from the Z-axis 46 with a positive value. Here, the fifth incident angle a 5 is equal to the value of the second incident angle. The fifth incident ray 90 is refracted when entering into the Fresnel lens 40 and becomes the fifth refracted ray 92. The fifth refracted ray 92 is further refracted when leaving the Fresnel lens 40, thus becomes the fifth emitted ray 94. A fifth emission angle β δ of the fifth emitted ray 94, which is also measured from the Z- axis 46, is smaller than the fifth incident angle a 5 . Here, the value of the fifth emission angle β§ is equal to the value of the second emission angle β 2 . [0067] In a like manner, the sixth ray 32 is located further to the right of the fifth ray 30 and is composed of a sixth incident ray 96, a sixth refracted ray 98 and a sixth emitted ray 100. These rays 96, 98, 100 are sequentially connected, coming from the centre point O. According to Figure 1 , a sixth incident angle a 6 is measured from the Z-axis 46 with a positive value. Here, the sixth incident angle a 6 is equal to the value of the third incident angle a 3 . The sixth incident ray 96 is refracted when entering into the Fresnel lens 40 and becomes the sixth refracted ray 98. The sixth refracted ray 98 is further refracted when leaving the Fresnel lens 40, thus becomes the sixth emitted ray 100. A sixth emission angle ββ of the sixth emitted ray 100, which is also measured from the Z-axis 46, is smaller than the sixth incident angle a 6 . Here, the sixth emission angle ββ is equal to the third emission angle β 3 . [0068] In Figure 1 , the first ray 22, the second ray 24 and the third ray 26 are selected in such a way that the third incident angle a 3 has three times the value as the first incident angle a^. The second incident angle a 2 is two times the value of the first incident angle o^. Similarly, the sixth incident angle a 6 32 has three times the value as the fourth incident angle a 4 . The fifth incident angle a 5 is two times the value of the fourth incident angle a 4 . Remote ends (not shown) of the second ray R 2 24 and the fourth ray R 4 30 define lateral boundaries of an illumination area at a distance for photograph. Hence, the third ray R3 26 and the sixth ray R6 32 are unable to contribute to the illuminated area at the distance. [0069] Figure 2 is a diagram illustrating an isometric view of the Fresnel lens 40 of the flash unit 20 for showing its Fresnel zones 62. The Fresnel zones 62 are visible in Figure 1 as discrete steps or teeth of an uneven surface. According to Figure 2, the Fresnel lens 40 comprises twelve Fresnel zones 102-124, which are sequentially adjoined from left to eight, including a Fresnel zone A 102, a Fresnel zone B 104, a Fresnel zone C 106, a Fresnel zone D 108, a Fresnel zone E 110, a Fresnel zone F 112, a Fresnel zone G 114, a Fresnel zone H 116, a Fresnel zone J 118, a Fresnel zone K 120, a Fresnel zone L 122 and a Fresnel zone M 124. The optical centerline 46 divides the twelve Fresnel zones 102-124 into two groups that are symmetrical with respect to the optical centerline 46. Widths 126 of each of these Fresnel zones 102-124 are substantially equal in a longitudinal direction (A'-axis) of the Fresnel lens 40, although only one of such widths is marked in Figure 2. The width of these Fresnel zones 102-124 is known as a pitch 126 of the Fresnel lens 40 such that an equivalent convex lens (not shown) is divided in the width direction (A ' -axis direction) to form these Fresnel zones 102-124. Lengths 130 of these Fresnel zones 102-124 are substantially the same in lateral direction (K-axis) of the Fresnel lens 40. However, heights of these Fresnel zones 102-124 are different, which are in a direction following the Z-axis 46. Uneven surfaces of the Fresnel zones 102-124 are located on a top surface 125 (internal/interior side) of the Fresnel lens 40, whilst a bottom surface 127 (exposed side) of the Fresnel lens 40 is smooth and flat. [0070] In detail, the Fresnel zone F 112 and the Fresnel zone G 114 are contiguous to each other. They 112, 14 are both next to the optical centerline 46 and are at opposite sides of the optical centerline 46. The Fresnel zone E 110 is contiguous to the Fresnel zone F 112 to the left, whilst the Fresnel zone H 116 is contiguous to the Fresnel zone G 114 to the right. The Fresnel zone E 110 and the Fresnel zone H 116 are symmetrical with respect to the optical centerline 46 too. Similarly, the Fresnel zone D 108 is contiguous to the Fresnel zone E 110 further to the left, and the Fresnel zone J 118 is contiguous to the Fresnel zone H 116 further to the right. The Fresnel zone D 108 and the Fresnel zone J 118 are symmetrical with respect to the optical centerline 46 as well. In a like manner, the Fresnel zone C 106 is contiguous to the Fresnel zone D 108 to the left, and the Fresnel zone K 120 is contiguous to the Fresnel zone J 118 to the right. The Fresnel zone C 106 and the Fresnel zone K 120 are symmetrical with respect to the optical centerline 46. Accordingly, the Fresnel zone B 104 is contiguous to the Fresnel zone C 106 to the left, and the Fresnel zone L 122 is contiguous to the Fresnel zone K 120 to the right. The Fresnel zone B 104 and the Fresnel zone L 122 are symmetrical with respect to the optical centerline 46. Lastly, the Fresnel zone A 102 is contiguous to the Fresnel zone B 104 to the left, and the Fresnel zone M 124 is contiguous to the Fresnel zone L 122 to the right. The Fresnel zone A 102 and the Fresnel zone M 124 are symmetrical with respect to the optical centerline 46. [0071] Figure 2 indicates that each of the Fresnel zones 102-124 has the width/pitch 126 substantially equal to 0.5mm (millimeter). The width is also known as a size of the teeth/steps, although a dimension/length in Y-axis direction of the Fresnel zones 102- 124 is much longer than the pitch. Figure 2 also shows that an overall width 28 of the Fresnel lens 40 is about 6 millimeter. Each of the Fresnel zones A 102 & M 124 has a height 132 of about 0.069mm, which is also known as the maximum material removal of the Fresnel lens 40. An overall height (thickness) 129 of the Fresnel lens 40, which is in the Z-axis 46 direction, is about 0.5mm. A length 130 of the Fresnel lens 40 is about 3mm, which lies in a lateral direction of the Fresnel lens 40. [0072] Figure 2 further presents an outline 134 of the Fresnel lens 40. The outline 134 comprises a portion 136 of a circle whose radius 138 is about 20mm. The radius 138 is alternatively known as an equivalent radius 138 of the Fresnel lens 40. The outline 134 gives a cross section of a plano-convex lens that has equivalent power (refraction effect) of the Fresnel lens 40. Hence, the portion 136 on top of the outline 34 is also known as an equivalent profile/surface 136 representing a curvature of the planoconvex lens. Referring Figures 1 & 2 together, the overall thickness of the Fresnel lens 40 is much thinner than the equivalent plano-convex lens because these Fresnel zones A-M 102-124 replace a bulk volume of the equivalent plano-convex lens. In other words, the Fresnel zones 102-124 effectively divide a continuous surface of the plano-convex lens into surface sections of the same curvature, with stepwise discontinuities between them. In fact, the Fresnel lens 40 may be regarded as an array of prisms arranged in a linear fashion, with steeper prisms on the edges and a nearly flat convex center. [0073] Figure 3 is a diagram illustrating a first known lamp cover 140 that incorporates the Fresnel lens 40. The lamp cover 140 has more Fresnel zones than those of the Fresnel lens 40 of Figure 1 & 2 because Figures 1 & 2 are schematic drawings with less number of Fresnel zones than those of Figure 3 for easier illustration. The lamp cover 140 incorporates the Fresnel lens 40 that protrudes on its top. The bottom surface 127 of the Fresnel lens 40 is exposed on top of the lamp cover 140. The lamp cover 140 has a thickness 142 of about 3mm (excluding the thickness of the protruding Fresnel lens 40), a width 144 of about 22mm in X-axis direction and a length 146 of about 4mm in Y-axis direction. The lamp cover 140 is made of transparent acrylic material and fits into the casing 42 for being installed into a mobile phone handset. [0074] Figure 4 is a diagram illustrating another known lamp cover 148 (second known lamp cover or Fresnel lens) with an outline 150. An equivalent radius 152 of the Fresnel lens 148, which starts from a vertex of the equivalent profile 151 and ends on the equivalent profile 151 , is about 5mm. Similar to the Fresnel lens 40, the Fresnel lens 148 has twelve discrete Fresnel zones N-Z 154-176. The left six Fresnel zones N-T 154-164 are symmetrically distributed on an opposite side of the right Fresnel zones 166-176 with respect to the -axis 46. The twelve Fresnel zones 154- 176 consist of a Fresnel zone N 154, a Fresnel zone P 156, a Fresnel zone Q 158, a Fresnel zone R 160, a Fresnel zone S 162, a Fresnel zone T 164, a Fresnel zone U 166, a Fresnel zone V 168, a Fresnel zone W 170, a Fresnel zone X 172, a Fresnel zone Y 174 and a Fresnel zone Z 176 that are sequentially and contiguously connected to each other. Uneven surfaces of the Fresnel zones 154-176 are positioned on a top side 177 of the Fresnel lens 148, whist a bottom side 179 of the Fresnel lens 148 is flat and smooth. These Fresnel zones 154-176 have substantially the same width 178 of about 0.5mm, which is also known as a pitch. A maximum material removal 180 of the second Fresnel lens 148, which occurs at the outermost of the Fresnel lens 148, is about 0.33mm. An overall height 182 of the second Fresnel lens 148, alternatively termed a thickness 182, is about 0.5mm. [0075] The equivalent profile 151 presents an external profile of a plano-convex lens that has equivalent power (refraction effect) of the Fresnel lens 148. In particular, the outline 150 comprises the equivalent profile 151 as a part of a circle whose radius is about 5mm. The equivalent profile 151 represents curvature of the plano-convex lens. [0076] Comparing the Fresnel lens 40 with the Fresnel lens 148, the Fresnel lens 148 can refract rays more effectively than the Fresnel lens 40. In other words, emission angles (not shown) of the second Fresnel lens 148 are smaller than those of the Fresnel lens 40 respectively if the incident angles αι -6 of incoming rays are the same. However, the second Fresnel lens 148, which is made of acrylic material, is overly fragile because a thinnest part of the second Fresnel lens 148 is about 0.17mm only. [0077] Figure 5 is a diagram illustrating a second known lamp cover 186. The lamp cover 186 incorporates a third known Fresnel lens 188 whose pitch is about 0.5mm (not marked). In contrast with the other known Fresnel lenses 40, 148, the Fresnel lens 188 has circular Fresnel zones around their optical centerline Z-axis 46. The transparent lamp cover 186 has a width of 22mm 190 in X-axis direction, a length 192 of 4mm in Y-axis direction and a thickness 194 of 3mm in Z-axis direction. [0078] Figures 6 and 7 relates to a first embodiment of the present application. These figures and their corresponding description include parts that are similar to those of previously described. The similar parts may be labeled with similar reference numerals. Description of the similar parts is thus incorporated by reference where appropriate. [0079] Figure 6 is a diagram illustrating a cross-section view of a first flash unit 200. The first flash unit 200 has a flash lamp 34, two reflectors 36, 38 and a first lamp cover 202. The first lamp cover 202 has an array of Fresnel zones on an inner side 63 such that the first lamp cover 202 can refract light as a Fresnel lens. Accordingly, the first lamp cover 202 is known as a first Fresnel lens 202. Figure 6 further shows an optical centerline 46 of the first flash unit 200, marked as the Z-axis 46. The optical centerline 46 passes through a centre point O of the flash lamp 34. Figure 6 further indicates six representative rays 204-214 that originate from the centre point O. The six rays 204-214 consist of a seventh ray 204, an eighth ray 206, a ninth ray 208, a tenth ray 210, an eleventh ray 212 and a twelfth ray 214. [0080] In detail, the seventh ray 204 consists of a seventh incident ray 216, a seventh refracted ray 218 and a seventh emitted ray 220. The seventh incident ray 216 coincides with the first incident ray 66 in their traces. The three rays 216, 218, 220 are sequentially connected, starting from the centre point O. Refraction of the seventh incident ray 216, which occurs when passing through the glass tube 48, is omitted due to its little impact to the overall optical effect. According to Figure 6, a seventh incident angle a 7 is measured from the Z-axis 46 with a positive value. Here, the seventh incident angle a 7 has the same value as that of the first incident angle ai and the fourth incident angle a . The seventh incident ray 216 is refracted when entering into the first Fresnel lens 202 and becomes the seventh refracted ray 218. The seventh refracted ray 218 is further refracted when leaving the first Fresnel lens 202, thus becomes the seventh emitted ray 220. A seventh emission angle β of the seventh emitted ray 204 is measured from the Z-axis 46. [0081] The eighth ray 206 is on the left side of the seventh ray 204 and the eighth ray 206 consists of an eighth incident ray 222, an eighth refracted ray 224 and an eighth emitted ray 226. The eighth incident ray 222 coincides with the second incident ray 72 in their paths. These three rays 222, 224, 226 are sequentially connected, starting from the centre point O. Refraction of the eighth incident ray 222, which occurs when passing through the glass tube 48, is omitted due to its little impact without affecting overall refraction. According to Figure 6, an eighth incident angle a 8 is measured from the Z-axis 46 with a positive value. Here, the eighth incident angle a 8 has the same value as that of the second incident angle c<2 and the fifth incident angle a 5 . The eighth incident ray 222 is refracted when entering into the first Fresnel lens 202 and becomes the eighth refracted ray 224. The eighth refracted ray 224 is further refracted when leaving the first Fresnel lens 202, thus becomes the eighth emitted ray 226. An eighth emission angle β 8 of the eighth emitted ray 226 is measured from the Z-axis 46. [0082] The ninth ray 208 is located further to the left of the eighth ray 206 and the ninth ray 208 consists of a ninth incident ray 228, a ninth refracted ray 230 and a ninth emitted ray 232. The ninth incident ray 228 coincides with the third incident ray 78 in their traces. The three rays 228, 230, 232 are sequentially connected, starting from the centre point O. Refraction of the ninth incident ray 228, which occurs when passing through the glass tube 48, is omitted due to its little refractive effect. According to Figure 6, a ninth incident angle a 9 is measured from the Z-axis 46 with a positive value. Here, the ninth incident angle a 9 has the same value as that of the third incident angle a 3 and the sixth incident angle a 6 . The ninth incident ray 228 is refracted when entering into the first Fresnel lens 202 and becomes the ninth refracted ray 230. The ninth refracted ray 230 is further refracted when leaving the first Fresnel lens 202, thus becomes the ninth emitted ray 232. A ninth emission angle β θ of the ninth emitted ray 232 is measured from the Z-axis 46. [0083] The tenth ray 210, the eleventh ray 212 and the twelfth ray 214 are located on the right side of the Z-axis 46, in contrast to the seventh ray 204, the eighth ray 206 and the ninth ray 208. In particular, the tenth ray 210 is at the right side of the Z-axis 46 and consists of a tenth incident ray 234, a tenth refracted ray 236 and a tenth emitted ray 238. The tenth incident ray 234 coincides with the fourth incident ray 84 in their traces. The three rays 234, 236, 238 are sequentially connected, originating from the centre point O. Refraction of the tenth incident ray 210, which occurs when passing through the glass tube 48, is omitted due to its little refraction. According to Figure 6, a tenth incident angle a 0 is measured from the Z-axis 46 with a positive value. Here, the tenth incident angle α-ιο has the same value as that of the first incident angle CH, the fourth incident angle a 4 and the seventh incident angle a 7 . The tenth incident ray 234 is refracted when entering into the first Fresnel lens 202 and becomes the tenth refracted ray 236. The tenth refracted ray 236 is further refracted when leaving the first Fresnel lens 202, thus becomes the tenth emitted ray 238. A tenth emission angle βιο of the tenth emitted ray 204 is measured from the Z-axis 46. [0084] The eleventh ray 212 is at the right side of the tenth ray 210 and consists of an eleventh incident ray 240, an eleventh refracted ray 242 and an eleventh emitted ray 244. The eleventh incident ray 240 coincides with the fifth incident ray 90 in their traces. The three rays 240, 242, 244 are sequentially connected, starting from the centre point O. Refraction of the eleventh incident ray 240, which occurs when passing through the glass tube 48, is omitted due to its little impact without affecting overall refraction. According to Figure 6, an eleventh incident angle an is measured from the Z-axis 46 with a positive value. Here, the eleventh incident angle an has the same value as that of the second incident angle a 2 , the fifth incident angle a 5 and the eighth incident angle as. The eleventh incident ray 240 is refracted when entering into the first Fresnel lens 202 and becomes the eleventh refracted ray 212. The eleventh refracted ray 242 is further refracted when leaving the first Fresnel lens 202, thus becomes the eleventh emitted ray 244. An eleventh emission angle βϋ of the eleventh emitted ray 244 is measured from the Z-axis 46. [0085] The twelfth ray 214 is further at the right side of the eleventh ray 212 and consists of a twelfth incident ray 246, a twelfth refracted ray 248 and a twelfth emitted ray 250. The twelfth incident 246 coincides with the sixth incident ray 96 in their traces. The three rays 246, 248, 250 are sequentially connected, starting from the centre point O. Refraction of the twelfth incident ray 246, which occurs when passing through the glass tube 48, is omitted due to its little refractive effect. According to Figure 6, a twelfth incident angle a 12 is measured from the Z-axis 46 with a positive value. Here, the twelfth incident angle a 12 has the same value as that of the third incident angle a 3 , the sixth incident angle a 6 and the ninth incident angle a 9 . The twelfth incident ray 246 is refracted when entering into the first Fresnel lens 202 and becomes the twelfth refracted ray 248. The twelfth refracted ray 248 is further refracted when leaving the first Fresnel lens 202, thus becomes the twelfth emitted ray 250. A twelfth emission angle βι 2 of the twelfth emitted ray 250 is measured from the Z-axis 46. [0086] Figure 7 is a diagram illustrating an isometric view of the first lamp cover 202, which is a part of the first flash unit 200. The first lamp cover 202 has a cuboid form with a top region 252 that is covered by an array of teeth or steps 254. The teeth 254 are Fresnel zones 254 which enable the first lamp cover 202 to function as the first Fresnel lens 202. Remaining five sides of the cuboid are substantially flat and rectangular in shape. The Fresnel zones 254 are uniform in their widths such that they have the same pitch 255. The pitch 255 defines the limit/size of each of these Fresnel zones 254 at 0.07mm in an X-a\ s direction. A maximum material removal 257 (height) of the first Fresnel lens 202 is 0.053mm. [0087] An outline 256 of the first lamp cover 202 is depicted on top of the isometric view. The outline 256 comprises an equivalent profile 258 that corresponds to the Fresnel lens 202. The equivalent profile 258 has the lowest point near its optical centre, which corresponds to a centre 259 of the first Fresnel lens 202. The equivalent profile 258 follows a curvature of an aspheric lens or asphere whose surfaces is rotationally symmetric. The surface is defined by an even form of sag equation (4) for defining the aspheric surface with conic and high order polynomial coefficient. z(r) = + a,r 2 + 2 r 4 + a i r 6 + a 4 r 8 + 5 r w + cc 6 r u + 7 r >4 + s r (4) [0088] In the above equation (4), 2 is the sag representation of the equivalent profile 258; R is the equivalent radius of curvature of the surface; r is a radial coordinate; K \ ' S the conic coefficient; and a, to a % or more are high order aspheric coefficients. Here only even terms of the high order aspheric expansion are defined, which is a common optical design technique. Here, c may be introduced for replacing R in the sag equiation if c = 1/R because it is common to use c instead of R in optical software packages. [0089] [0090] The equivalent profile 258 uses a set of coefficients in the following Table. Particularly, κ and a, to a % are all zero and therefore equivalent profile 258 is a portion of the curvature of a sphere. The negative sign of equivalent radius R indicates equivalent profile 258 is concave. [0090] Comparing the first Fresnel lens 202 with the Fresnel lens 40, emission angles of first Fresnel lens 202 are larger than those of Fresnel lens 40. More specifically, the emission angles β 7 >βι , β8>β2, β9>β3, βιο>β4, βι ι>βδ and βΐ2>ββ, although incident rays of these two Fresnel lenses 202, 40 have the same incident angles respectively (i.e. The larger emission angles signify that the first Fresnel lens 202 refracts the same incident light more effectively than the Fresnel lenses 40. In other words, the emitted rays R7-R12 of the Fresnel lens 202 are more divergent than those of the Fresnel lens 40. The larger divergence is caused by the concave portion of the equivalent profile 258 such that a larger area can be covered by the flash unit 202 using the first Fresnel lens 202. The larger area coverage produces more uniform distribution of flashlight for photography and is more favorable for wide-angle photograph. [0091] Figure 8 is a diagram illustrating a diamond cutter 260 in a process of cutting a mould insert 262 for making the first Fresnel lens 202. A tip of the diamond cutter 262 has a radius (marked as R) of 5 microns (R=0.005mm or 5μηη). The first Fresnel lens 202 is mass-produced by a plastic injection moulding technique using methyl methacrylate (PMMA). [0092] According to Figure 8, the diamond cutter 260 has finished the cutting of a first Fresnel tooth (Fresnel zone or optical zone) 272 of the mould insert 262, as indicated by a solid line A-B-C. A second Fresnel tooth 274, which is yet to be cut, is defined by a dash line D-E-F. The mould insert 262 has corners B & E with a radius of 5 microns, conforming to the radius of the tip of the diamond cutter 262. The diamond cutter 260 is in a process of moving to a next position 264, which is depicted a dash line. A base surface 266 of the mould insert 262 is marked by a line H'-H. [0093] Figure 8 further indicates that continuation surfaces 268, 270 between neighbouring Fresnel teeth 272, 274 are not exactly perpendicular to the base surface 266. In particular, a second continuation surface 270 between a second Fresnel zone surface 274 on the right and another Fresnel zone (tooth) surface on the left (not shown) has a draft angle Θ of 4.5°. Similarly, a first continuation surface 268 between a first Fresnel zone surface 272 on the right and the second Fresnel zone surface 274 on the left also has a draft angle of 4.5°. The diamond cutter 260 moves from the corner B to the next position 264 for cutting the first continuation surface 268. The draft angle Θ enables easy ejection of the moulded first Fresnel lens 202 after cooling down such that the freshly moulded first Fresnel lens 202 is not deformed during the ejection. [0094] The moulding material of methyl methacrylate (PMMA) can alternatively be replaced by polystyrene, polycarbonate (PC) or other optical plastic materials. The mould insert or mould can also be prepared by stainless steel material. Typically, the copper mould insert can withstand 200,000 moulding cycles, whilst the stainless steel mould insert can sustain 1 ,000,000 moulding cycles without being re-sharpened. The tip of the diamond cutter 260 can be made sharper, having a radius of from 1 micron to 5 microns. Normally, if mould insert 262 is cut with the diamond cutter having a tip of radius 1 to 5 microns by a horizontal feeding speed of 1 micron per step, the first Fresnel lens 202 can have good surface finish quality for excellent optical performance. Cutting efficiency of the mould insert 262 is also acceptable by using the feeding speed of one micron (0.001 mm) per step of feeding. [0095] Figures 9 and 10 relate to a second embodiment. The second embodiment contains parts that are similar to those described above. The similar parts are labeled by the same or similar reference numerals. Description of the similar parts is thus incorporated by reference where appropriate. [0096] In particular, Figure 9 is a diagram illustrating a cross section view of a second flash unit 276. The second flash unit 276 comprises a flash lamp 34, two reflectors 36, 38 and a second lamp cover 278 that are enclosed by a casing 42. Six representative rays R13-R18 280-290 are symmetrically plotted beside a Z-axis 46 in Figure 9. The second lamp cover 278 comprises Fresnel zones 330 on its inner side 63 such that the second lamp cover 278 is also known as a second Fresnel lens 278. [0097] The six rays 280-290 are emitted from the centre (marked as Point "0") of the flash lamp 40 and pass through the second Fresnel lens 278. The thirteenth ray 280, the fourteenth ray 282 and the fifteenth ray 284 are symmetrically disturbed against the sixteenth ray 286, the seventeenth ray 288 and the eighteenth ray 290 with respect to the optical centerline Z-axis 46. For simplicity, rays that land on and depart from the two reflectors 36, 38 are omitted. [0098] In particular, the thirteenth ray 280 is positioned at the left of the Z-axis 46 and is composed of a thirteenth incident ray 292, a thirteenth refracted ray 294 and a thirteenth emitted ray 296. The three rays 292, 294, 296 are sequentially connected, starting from the centre point O. Refraction of the thirteenth incident ray 292, which occurs when passing through the glass tube 48, is omitted due to its little effect without affecting overall refraction. According to Figure 9, a thirteenth incident angle a 3 is measured from the Z-axis 46 with a positive value. Here, the thirteenth incident ray 292 coincides with the seventh incident ray 216 such that the thirteenth incident angle ai 3 has the same value as that of the seventh incident angle a 7 . The thirteenth incident ray 280 is refracted when entering into the second Fresnel lens 278 and becomes the thirteenth refracted ray 294. The thirteenth refracted ray 294 is further refracted when leaving the second Fresnel lens 278, thus becomes the thirteenth emitted ray 296. A thirteenth emission angle βΐ3 of the thirteenth emitted ray 296, which is also measured from the Z-axis 46, is smaller than the seventh emission angle β 7 . [0099] Similarly, the fourteenth ray 282 is located further to the left of the thirteenth ray 280 and is composed of a fourteenth incident ray 298, a fourteenth refracted ray 300 and a fourteenth emitted ray 302. These rays 298, 300, 302 are sequentially connected, starting from the centre point O. The fourteenth incident ray 298 coincides with the eighth incident ray 222 such that a fourteen incident angle α-ι 4 and the eighth incident angle a 8 are of identical size. The fourteenth incident ray 298 is refracted when entering into the second Fresnel lens 278 and becomes the fourteenth refracted ray 300. The fourteenth refracted ray 300 is further refracted when leaving the second Fresnel lens 278, thus becomes the fourteenth emitted ray 302. A fourteenth emission angle βι 4 of the fourteenth emitted ray 302, which is also measured from the Z-axis 46, is smaller than the eighth emission angle ββ. [0100] In a like manner, the fifteenth ray 284 is located further to the left of the fourteenth ray 282 and is composed of a fifteenth incident ray 304, a fifteenth refracted ray 306 and a fifteenth emitted ray 308. These rays 304, 306, 308 are sequentially connected, starting from the centre point O. According to Figure 9, the fifteenth incident ray 304 coincides with the ninth incident ray 228 such that a fifteenth incident angle a-| 5 is the same as the ninth incident angle a 9 . The fifteenth incident ray 304 is refracted when entering into the second Fresnel lens 278 and becomes the fifteenth refracted ray 306. The fifteenth refracted ray 306 is further refracted when leaving the second Fresnel lens 282, thus becomes the fifteenth emitted ray 308. A fifteenth emission angle β 15 of the fifteenth emitted ray 308, which is also measured from the Z-axis 46, is smaller than the ninth emission angle β θ . [0101] The other three rays 286-290 are positioned on an opposite side of the Z-axis 46. In detail, the sixteenth ray 286 is located to the right of the Z-axis 46 and is composed of a sixteenth incident ray 310, a sixteenth refracted ray 312 and a sixteenth emitted ray 314. The three rays 310, 312, 314 are sequentially connected, beginning from the centre point O. Refraction of the sixteenth incident ray 310, which occurs when passing through the glass tube 48, is omitted due to its negligible refractive effect. According to Figure 9, a sixteenth incident angle c<i6 is measured from the Z-axis 46 with a positive value. Here, the value of Che is equal to the value of α-ι, a 4 , a 7 , a-io, and 0:13 respectively. The sixteenth incident 310 is refracted when entering into the second Fresnel lens 278 and becomes the sixteenth refracted ray 312. The sixteenth refracted ray 312 is further refracted when leaving the second Fresnel lens 278, thus becomes the sixteenth emitted ray 314. A sixteenth emission angle βιβ of the sixteenth emitted ray 288, which is also measured from the Z-axis 46, is smaller than the fourth emission angle β 4 or the tenth emission angle βι 0 . [0102] Similarly, the seventeenth ray 288 is located further to the right of the sixteenth ray 286 and is composed of a seventeenth incident ray 316, a seventeenth refracted ray 318 and a seventeenth emitted ray 320. These rays 316, 318, 320 are sequentially connected, originating from the centre point O. As shown in Figure 9, a seventeenth incident angle a 17 is measured from the Z-axis 46 with a positive value. Here, the seventeenth incident angle ai is equal to the value of the second incident angle a 2 . The seventeenth incident ray 316 is refracted when entering into the second Fresnel lens 278 and becomes the seventeenth refracted ray 318. The seventeenth refracted ray 318 is further refracted when leaving the second Fresnel lens 278, thus becomes the seventeenth emitted ray 320. A seventeenth emission angle β·ι 7 of the seventeenth emitted ray 320, which is also measured from the Z-axis 46, is smaller than the eleventh emission angle an. Here, the value of the seventeenth emission angle βι is the same as the value of the fourteenth emission angle βι 4 . [0103] In a like manner, the eighteenth ray 290 is located further to the right of the seventeenth ray 288 and is composed of an eighteenth incident ray 322, an eighteenth refracted ray 324 and an eighteenth emitted ray 326. These rays 322, 324, 326 are sequentially connected, starting from the centre point O. According to Figure 9, an eighteenth incident angle Qis is measured from the Z-axis 46 with a positive value. Here, the eighteenth incident angle a 18 is equal to the value of the fifteenth incident angle 015. The eighteenth incident ray 322 is refracted when entering into the second Fresnel lens 40 and becomes the eighteenth refracted ray 324. The eighteenth refracted ray 324 is further refracted when leaving the second Fresnel lens 278, thus becomes the eighteenth emitted ray 326. An eighteenth emission angle β 18 of the eighteenth emitted ray 326, which is also measured from the Z-axis 46, is smaller than twelfth emission angle βι 2 . Here, the eighteenth emission angle βι 8 is equal to the fifteenth emission angle Pis ¬ to 04] Figure 10 is a diagram illustrating an isometric view of the second Fresnel lens 278, which is a part of the second flash unit 276. The second Fresnel lens 278 is also in the form of a cuboid. The second Fresnel lens 202 has a top region 328 that is covered by the array of Fresnel zones 330 in a linear manner. The Fresnel zones 330 are teeth or steps that form an uneven surface. Remaining five sides of the cuboid are substantially flat and rectangular in shape. An outline 332 of the second Fresnel lens 278 is depicted on top of the isometric view. The outline 332 comprises an equivalent profile 334 that corresponds to the teeth of the second Fresnel lens 278. [0105] The equivalent profile 334 follows the curvature of an aspheric lens or asphere whose surfaces is rotationally symmetric, but is not a portion of a sphere. The surface is defined by the sag equation (4) with a set of coefficients for describing an aspheric surface with conic and high order polynomial coefficient as shown in the following table. [0106] Figure 10 is a diagram illustrating that the equivalent profile 334 has a peak 336 that is located between two valleys 337, 339. The peak 336 is also known as a convex portion 336 of the second Fresnel lens 278, whilst the portion 338 at the left side of the valley 337, and the portion 340 at the right side of valley 339 are also known as concave portions of the second Fresnel lens 278. The convex portion 336 is located at a centre 259 of the second Fresnel lens 278, whilst the concave portions are located at opposite ends 341 , 343. The first concave portion is near a first end 341 of the second Fresnel lens 278 and the second concave portion 340 is near the second end 343 of the Fresnel lens 278. Similar to the equivalent profile 258 of the first Fresnel lens 202, the equivalent profile 334 of the second Fresnel lens 278 is also affected by the values of c, k, r and a 1--8 . Relative positions and gradients of the convex portion 336, a first concave portion 338 and a second concave portion 340 can be adjusted by varying these coefficients too. Figure 10 also shows that a height (maximum material removal) 344 for the second Fresnel lens 278 is about 0.053mm and a width/pitch 342 of the second Fresnel zone is about 0.1 mm. [0 07] Particularly, the positions and gradients of the concave portions 338, 340 and the convex portion 336 determine a desirable guide number achievable by the second flash unit 276 using the second Fresnel lens 278. To have more uniform light distribution over a flash light coverage area, the valleys 337, 339 can be brought closer to a lens centre near the Z-axis 46 such that the concave portions 338, 340 are extended. Alternatively, the gradient of the convex portion 336 can be made less or more gentle. The gradients of the concave portions 338, 340 can be made greater as a further alternative. A combination of any of these three approaches can be used to achieve more even light distribution over the flash coverage area by using aspheric profile. The concave portions 338, 340 with larger area or greater gradients are able to direct more light towards edges of the coverage area. The convex portion 336 with a lesser gradient (gentler gradient) is also capable of diverting more light towards the edges. Hence, the second Fresnel lens 278 that has a higher order of polynomial equivalent profile provides more flexibility in Fresnel lens design for desired illumination results. If a higher guide number with compromised illumination distribution is desired for the second flash unit 276, the concave portions 338, 340 may be reduced. [0108] Comparing Figure 9 and Figure 1 , both β 13 and β 16 are smaller than or β 4 respectively. Both β 4 and βι 7 are smaller than β 2 or β 5 respectively. βι 5 and βι 8 are significantly larger than β 3 or β 6 respectively. Hence, the second Fresnel lens 278 can focus more flash light towards the centre of flash light coverage area, although there is loss of light for illuminating because the fifteenth emitted ray 308 and the eighteenth emitted ray 326 are more divergent than the third emitted ray 82 and the sixth emitted ray 100. Referring to the Fresnel lens 40, the third ray R3 26 and the sixth ray R6 32 in Figure 1 are unable to contribute to the illuminated area at the distance. [0109] Figure 11 is a diagram illustrating an isometric view of a third lamp cover 346. The third lamp cover 346 has Fresnel zones 347 on its inner side 348 such that the third lamp cover 346 is also known as a third Fresnel lens 346. The third Fresnel lens 346 is a slab of acrylic with its inner side 348 on top, known as a top region 348. The top region 348 comprises the Fresnel zones 347 in the form of an array of teeth or steps. A height 350 (maximum material removal) of the Fresnel zones is 0.048mm. Each of the Fresnel zones has substantially the same width/pitch 352 in X-axis direction, which is about 0.1 mm. The third Fresnel lens 346 has a similar equivalent profile as that of the second Fresnel lens 278 with two concave portions 338, 340 and one convex portion 336. It is an example for more uniform light distribution over a flash light coverage area compared to Fresnel lens 278 by extending the concave portion 338 and 340, or decreasing the gradient of the convex portion 336, or further alternatively increasing the gradients of the concave portions 338, 340, or using the combinations of any of these three approaches. The third lamp cover 346 has guide numbers and light distribution (illumination results) similar to the known lamp covers 40 of Figure 2. However, the third lamp cover 346 is made much thinner than the known lamp covers 40 because the Fresnel zones 347 of the third lamp cover 346 uses a much smaller pitch. [0110] An outline 354 of the third Fresnel lens 346 is provided above the isometric projection of the third Fresnel lens 346. The outline 354 comprises an equivalent profile 356 of the third Fresnel lens 346. The equivalent profile 356 follows the sag equation (4) for defining the aspheric surface of the third Fresnel lens 346. Coefficients of the third Fresnel lens 346 using the coefficients in the following table. [0111] Figure 12 is a diagram illustrating an isometric view of a fourth lamp cover 358. The fourth lamp cover 358 is substantially a rectangular prism with a top region 360. The top region 360 comprises a group of linear Fresnel zones 361. Shapes of the Fresnel zones 361 resemble to teeth or steps. A height 363 (maximum material removal) of the Fresnel zones is 0.053mm. Each of these Fresnel zones has substantially the same pitch 362, which is about 0.1 mm. [0112] An outline 362 of the fourth Fresnel lens 358 is provided above the isometric projection of the fourth Fresnel lens 358. The outline 362 comprises an equivalent profile 364 of the fourth Fresnel lens 358. The equivalent profile 364 follows the sag equation (4) for defining aspheric surface of the fourth Fresnel lens 358 using the coefficients in following table. [0113] In contrast to the second and the third Fresnel lenses 278, 346, the equivalent profile 364 has two convex portions 368, 370 and one concave portion 366. The concave portion 366 is located between a first convex portion 368 and a second convex portion 370. [0 14] Figure 13 is a diagram illustrating a chart 372 for showing relative illuminance on flash coverage area versus flash coverage angle at a given time during the flash. The chart 372 is produced by employing an optical simulation software package. A horizontal axis 374 of the chart 372 is labeled by a flash coverage angle, which is known as the emission angle. A vertical axis 376 of the chart 372 is marked by a ratio of relative illuminance. The illuminance is measured in a length direction across the flash coverage area. For comparison, using the first known Fresnel lens 40 illuminance at a centre of the flash coverage area is 1.00 after normalization. The normalization transforms data of different groups using a same scale in order for easy comparison. In the chart 372, illuminance of all curves is plotted with referenced to 1.0. Integrating the illuminance at the flash coverage center over time in the flash duration is directly related to guide number. [0115] The chart 372 includes four curves 378, 380, 382, 383, 384 that indicate illuminance of the five Fresnel lenses 278, 346, 40, 358, 202 with the same light source 34 (flash lamp). In particular, a first curve 378 provides illuminance of the second Fresnel lens 278. A second curve 380 describes illuminance of the third Fresnel lens 346. A third curve 382 depicts illuminance of the first known Fresnel lens 40. A fourth curve 383 shows illuminance of the fourth Fresnel lens 358. A fifth curve 384 indicates illuminance of the first Fresnel lens 202. [0 16] These five curves 378, 380, 382, 383, 384 shows that the flash light coverage is the most uniform when using the first Fresnel lens 202. In other words, the first Fresnel lens is more applicable for wide angle photograph. In contrast, the second Fresnel lens 278 produces a brightest central area of illumination with the flash coverage area such that the relative illuminance reaches 1.1 at the centre area. The second Fresnel lens 278 is able to produce further higher relative illuminance using the methods discussed above (e.g. Para [0108]). The second Fresnel lens provides an extreme case for increasing the relative illuminance that the two valleys 337, 339 are moved towards the first end 341 and the second end 343 of the Fresnel lens 278 respectively so that the equivalent profile 334 contains only the convex portion 336. Further increasing the gradient of the convex portion 336 can additionally increase the relative illuminance. Higher relativeO luminance means a higher guide number and is more applicable for telephoto photograph. The third Fresnel lens 346 produces a similar illumination pattern as that of the Fresnel lens 40. [0117] Figure 14 is a diagram illustrating a transparent lamp cover 386 with varying sizes of Fresnel zones 388, 390, 392. The transparent lamp cover 386 incorporates circular Fresnel zones 388, first linear Fresnel zones 390 and second linear Fresnel zones 392. The Fresnel zones 388, 390, 392 are provided on an interior surface of the transparent lamp cover such that an exposed side 64 (exterior surface) is flat and smooth. The circular Fresnel zones 388 comprise concentric annular sections, which are equivalent to a convex lens divided into discrete Fresnel zones. The first linear Fresnel lens 390 and the second linear Fresnel lens 392 are symmetrical with respect to a centre of the circular Fresnel zones 388. More particularly, the first linear Fresnel zones 390 comprise parallel teeth with a decreasing pitch towards an edge of the transparent lamp cover 386. Near to the circular Fresnel zones 388, the first linear Fresnel zones 390 have a width/size of 0.5mm. Farthest from the circular Fresnel zones 388, the first linear Fresnel zones have a width/size of 0.04mm. The second linear Fresnel zones 392 are symmetrical to the first linear Fresnel zones 390. Both the circular Fresnel zones 388 and the linear Fresnel zones 390, 392 have a substantially constant height/depth (maximum material removal) at about 0.04mm. The transparent lamp cover 386 provides a transparent lamp cover that combines both circular and linear Fresnel zones/teeth together. [0118] Figure 15 is a diagram illustrating another transparent lamp cover 394 with densely allocated Fresnel zones 396. The Fresnel zones 396 are circular in nature and are provided on an interior side (not visible) of the transparent lamp cover 394 with a uniform pitch of 0.07mm. Since a naked human eye is unable to differentiate clearly a slit of 0.07mm or less placed at a point of most distinct vision, the Fresnel zones 396 appears to be almost an opaque or reflective patch to the naked eye, which is shown in Figure 15. The transparent lamp cover 394 is thus used to seal a window 43 of the flash unit 200 for concealing internal components. Hidden components of the flash unit 200 include reflectors 36, 38, flash lamp 48 and wires. [0119] More particularly, a central area 398 comprises four quadrants 400, 402, 404, 406. A top quadrant 400 has a similar pattern as that of a bottom quadrant 402. Near a centre point 408 of the transparent lamp cover 394, the two quadrants 400, 402 have two areas that have Fresnel zones of pitch 0.5mm. On the other hand, a left quadrant 404 has a similar pattern as that of a right quadrant 406. The left quadrant 404 and the right quadrant 406 are symmetrical with respect to the centre point 408 of the transparent lamp cover 394. These two quadrants 404, 406 also have two areas that have Fresnel zones of pitch 0.5mm. Away from the Fresnel zones of pitch 0.5mm, each of the four quadrants 400, 402, 404, 406 has circular regions with fine Fresnel zones. The transparent lamp cover 394 of Figure 15 presents a unique appearance for visually shielding an interior of a flash unit 200, 276, which is useful for a portable electronic device with the flash unit 200, 276. [0120] In the application, unless specified otherwise, the terms "comprising", "comprise", and grammatical variants thereof, intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, non-explicitly recited elements. [0121] As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1 % of the stated value, and even more typically +/- 0.5% of the stated value. [0122] Throughout this disclosure, certain embodiments may be disclosed in a range format. The description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1 , 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range. [0123] It will be apparent that various other modifications and adaptations of the application will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the application and it is intended that all such modifications and adaptations come within the scope of the appended claims. REFERENCE NUMERALS 20 first known flash unit 22 first ray 24 second ray 26 third ray 28 fourth ray 30 fifth ray 32 sixth ray 34 flash lamp 36 left reflector 38 right reflector 40 first known Fresnel lens 42 casing 43 window 44 back reflector 46 Z-axis 48 glass tube 50 left end 52 right end 54 left electrode 56 right electrode 58 interior 60 xenon gas 62 Fresnel zones 63 inner side 64 exposed side 66 first incident ray 68 first refracted ray 70 first emitted ray 72 second incident ray 74 second refracted ray 76 second emitted ray 78 third incident ray 80 third refracted ray 82 third emitted ray 84 fourth incident ray 86 fourth refracted ray 88 fourth emitted ray 90 fifth incident ray 92 fifth refracted ray 94 fifth emitted ray 96 sixth incident ray 98 sixth refracted ray 100 sixth emitted ray 102 Fresnel zone A 104 Fresnel zone B 106 Fresnel zone C 108 Fresnel zone D 110 Fresnel zone E 112 Fresnel zone F 114 Fresnel zone G 116 Fresnel zone H 118 Fresnel zone J 120 Fresnel zone K 122 Fresnel zone L 124 Fresnel zone M 125 top surface 126 width 127 bottom surface 128 width 129 height 130 length 132 height 134 outline 136 equivalent profile 138 radius 140 transparent lamp cover 142 thickness 144 width 146 length 148 second known Fresnel lens 150 outline 151 equivalent profile 152 radius 153 vertex 154 Fresnel zone N 156 Fresnel zone P 158 Fresnel zone Q 160 Fresnel zone R 162 Fresnel zone S 164 Fresnel zone T 166 Fresnel zone U 168 Fresnel zone V 170 Fresnel zone W 172 Fresnel zone X 174 Fresnel zone Y 176 Fresnel zone Z 177 top side 178 width 179 bottom side 180 height 182 thickness 84 equivalent profile 186 transparent lamp cover 188 third known Fresnel lens 190 width 192 length 94 thickness 200 flash unit 202 first lamp cover 204 seventh ray 206 eighth ray 208 ninth ray 210 tenth ray 212 eleventh ray 214 twelfth ray 216 seventh incident ray 218 seventh refracted ray 220 seventh emitted ray 222 eighth incident ray 224 eighth refracted ray 226 eighth emitted ray 228 ninth incident ray 230 ninth refracted ray 232 ninth emitted ray 234 tenth incident ray 236 tenth refracted ray 238 tenth emitted ray 240 eleventh incident ray 242 eleventh refracted ray 244 eleventh emitted ray 246 twelfth incident ray 248 twelfth refracted ray 250 twelfth emitted ray 252 top region 254 teeth 255 pitch 256 outline 257 maximum material removal 258 equivalent profile 259 centre 260 diamond cutter 262 mould insert 264 next position of diamond cutter 266 base surface 268 first continuation surface 270 second continuation surface 272 first Fresnel tooth 274 second Fresnel tooth 276 second flash unit 278 second lamp cover 280 thirteenth ray 282 fourteenth ray 284 fifteenth ray 286 sixteenth ray 288 seventeenth ray 290 eighteenth ray 292 thirteenth incident ray 294 thirteenth refracted ray 296 thirteenth emitted ray 298 fourteenth incident ray 300 fourteenth refracted ray 302 fourteenth emitted ray 304 fifteenth incident ray 306 fifteenth refracted ray 308 fifteenth emitted ray 310 sixteenth incident ray 312 sixteenth refracted ray 314 sixteenth emitted ray 316 seventeenth incident ray 318 seventeenth refracted ray 320 seventeenth emitted ray 322 eighteenth incident ray 324 eighteenth refracted ray 326 eighteenth emitted ray 328 top region 330 Fresnel zones 332 outline 334 equivalent profile 336 convex portion 337 first valley 338 first concave portion 339 second valley 340 second concave portion 341 first end 342 width 343 second end 344 height 346 third lamp cover 347 Fresnel zones 348 top region 350 height 352 width 354 outline 356 equivalent profile 358 fourth lamp cover 360 top region 361 Fresnel zones 362 outline 363 height 364 equivalent profile 366 concave portion 368 first convex portion 370 second convex portion 372 chart 374 horizontal axis 376 vertical axis 378 first curve 380 second curve 382 third curve 383 fourth curve 384 fifth curve 386 transparent lamp cover W 388 circular Fresnel zones 390 first linear Fresnel zones 392 second linear Fresnel zones 394 transparent lamp cover 396 Fresnel zones 398 central area 400 top quadrant 402 bottom quadrant 404 left quadrant 406 right quadrant 408 centre point Previous Patent: METAL NANOWIRES, NANOMESH AND A METHOD OF FABRICATIONNext Patent: A MEMBRANE SENSOR AND METHOD OF DETECTING FOULING IN A FLUID Privacy Policy | Terms of Use | ©