Light box

The light box is provided with a case having arranged therein a reflector directed to the front wall to be illuminated from the rear. The reflector surf ace has a concave curvature. An elongated light source is arranged in the center of the case. The reflector, in the central area thereof, is covered by an optical film having a structured surface directed to the light source. This structure consists of immediately adjacent V-shaped grooves, being parallel to each other and running transverse to the extension of the light source. By total reflection on the structured surface of the transparent optical film as well as by reflection on the reflector surface of the reflector, the light of the light source incident on the reflector is reflected in the direction of the back-lit front side of the light box. This arrangement provides for improved and more-uniform light distribution.

The invention is directed to a light box having a case with a back-lit 
front side, a light source for backlighting said front side, and a 
reflector with a curved reflector surface having provided thereon an 
optical film, said film, on the surface thereof directed towards the light 
source, being provided with substantially V-shaped grooves arranged side 
by side and in parallel to each other. 
The main problem with light boxes consists in that the front side or wall, 
carrying a signing, i.e. information, has to be illuminated evenly 
although a plurality of point sources of light or elongated sources of 
light (luminescent or fluorescent tubes) are used. Light boxes are 
installed wherever an information-carrying surface is to be back-lit by 
light. Therefore, light boxes are used primarily for back-lit posters, 
pictures, sign plates and the like. However, light boxes find also 
application as working place luminaries because they emit the light 
relatively evenly onto the working place. Further, light boxes are used 
for the backlighting of liquid crystal displays. 
A light box of the initially mentioned type is known from U.S. Pat. No. 
4,874,228. This known light box comprises a box substantially rectangular 
in section, with a (elongated) light source being arranged in one 
peripheral area of said box. Between the rear wall and the back-lit front 
side of the box opposite to the rear wall, there is arranged a reflector, 
having a reflector surface with a transparent optical film arranged 
thereon. Said optical film has an even surface showing towards the 
reflector surface and a structured surface provided with a plurality of 
substantially V-shaped grooves arranged side by side and in parallel to 
each other, i.e. a plurality of prisms arranged without mutual distances. 
Transparent optical films of this kind are also used for light 
transmission in air. In said known light box, the prisms or grooves run 
parallel to the extension of the light source, i.e. parallel to the tube. 
Depending on the respective angle of incidence, the beams incident on the 
structured surface of the optical film, traveling at relatively small 
angles of less than 20 degrees with respect to the even surface of the 
optical film, are either (totally) reflected or, while being refracted, 
penetrate into the optical film and are reflected on the reflector surface 
and then, after again penetrating the optical film, exit in the direction 
of the front side to be back-lit. The reflector, and thus also the optical 
film, does not extend in the immediate vicinity of the light source, i.e. 
is not arranged in the region near the light source. 
It is the object of the invention to provide a light box of 
initially-mentioned type which, while being simple in construction, 
provides almost completely even illumination of the front side. 
For solving said object, it is provided according to the invention that the 
light source is arranged in the central area of the reflector and that the 
V-shaped grooves are oriented transverse to the extension of the light 
source. 
In the light box of the invention, the reflector is arranged along both 
longitudinal sides of the elongated light source; thus, the reflector 
surface projects beyond the light source transverse to the longitudinal 
extension thereof. Accordingly, the light source is located in the central 
area of the reflector. The orientation or structure, respectively of the 
optical film on the reflector surface is such that the grooves or prisms 
run vertical to the longitudinal extension of the light source. The light 
beams, traveling in a relatively large range of angles of some 10 degrees 
to 90 degrees with respect to the even surface of the optical film, 
impinge on the structured surface thereof. The reflector (optical film) is 
curved and preferably has a varying radius of curvature. 
From the upper region of the light source, facing the front side, light is 
emitted directly onto the back-lit front side whereas from the lower 
region of the light source, being averted from the front side, light 
impinges onto the optical film of the reflector. Due to the orientation of 
the V-shaped grooves (or prisms), the light beams incident on the 
reflector are defracted in such a manner that they impinge on the front 
side after traveling past the side of the light source, i.e. without 
crossing it. The light beams emitted by the light source from the lower 
region thereof hit onto the oblique flanks of the V-shaped grooves or 
prisms where they are either reflected completely or, after being 
refracted, penetrate into the optical film and are reflected by the 
underlying reflector surface. By the structure of the optical film being 
oriented transverse to the extension of the light source, it is 
accomplished that those light beams which impinge on the reflector 
immediately near the light source are reflected past the side of the light 
source onto the front side and, therefore, do not add to the light aimed 
directly from the light source to the front side. As a result, there is 
provided a more uniform illumination of the front side in the central area 
thereof where the light from the light source impinges directly thereonto. 
This is particularly advantageous in flat light boxes since, due to the 
closeness of the front side to the light source, the front side tends to 
be illuminated more intensely in the area of the light source. This effect 
is evidently reduced in the light box of the invention. 
The optical film is flexible but is provided with some stability of shape, 
i.e. a certain bending resistance. Thus, the optical film substantially 
maintains its form and position when not being subjected to external 
forces. 
Basically, it is without relevance at which angle the flanks of the 
V-shaped grooves extend with respect to each other. Particularly favorable 
optical characteristics are obtained if the flanks of the optical film are 
oriented at a mutual angle of 90 degrees, with each flank arranged at an 
angle of 45 degrees relative to the even underside of the film. 
Preferably, the total thickness of the optical film is about 0.5 mm, the 
depth of the V-shaped grooves being about 0.17 mm and the width of the 
opening thereof being about 0.35 mm, respectively. Preferably, the 
material of the optical film consists of polycarbonate or 
polymethylmethacrylate. 
Preferably, an elongated lamp, particularly a fluorescent tube, is used as 
a light source. However, also a string of point sources of light is 
suitable. In the sense of the invention, an elongated light source is any 
light source with strip-shaped light radiation. Fluorescent tubes offer 
the advantage that light is emitted not just radially, as in a luminous 
filament, but (with the tube being seen from the side) from a relatively 
wide area of the strip, namely the area of the longitudinal section of the 
tube. Thereby, the angles of incidence of the light beams are equal or 
substantially equal over wide areas of the reflector, resulting in equal 
or similar reflections. Thus, a light source emitting light out of 
different distances to the reflector, as is the case with a tube, 
illuminates the reflector in a more uniform manner than a light source 
comprising a luminous filament. A more uniform illumination of the 
reflector, in turn, contributes to a more uniform illumination of the 
front side. 
Advantageously, the reflector surface under the optical film is provided 
for diffuse reflection; particularly, this surface is a mat white surface. 
On the surface, the portion of light entering into the optical film 
(depending on the angle of incidence of the light beams) is reflected 
diffusely, i.e. with a certain scattering or spreading. Preferably, the 
peripheral areas of the reflector, extending in parallel to the 
longitudinal dimension of the light source, are not covered by the optical 
film but are exposed. These peripheral areas reflect the light diffusely 
and thus add to the spreading of the light in the peripheral area of the 
front side. 
Preferably, the light source is arranged in the immediate vicinity of the 
structured surface of the optical film, e.g. in a distance of about 5 mm. 
In an advantageous embodiment of the invention, the curvature of the 
reflector surface in the area near the light source, i.e. in the central 
area, is stronger than in the remaining area of reflector surface. 
Preferably, the reflector surface is curved in a parabola. 
It should be considered that the uniform light distribution on the front 
side of the light box is effected primarily by reflection and refraction 
of the light on the optical film having the above-described surface 
structure. A light-scattering diffusion plate, as it is usually found in 
light boxes, is not needed at the front side. While in those light boxes 
which strictly require a diffusion plate for obtaining relatively uniform 
light distribution at the back-lit surface, comparatively large losses 
occur in the diffusion plate, this is not the case in the light box of the 
invention because here the uniform light distribution is not, or at least 
not primarily so, effected by a diffusion plate. Accordingly, light 
sources having low light intensity and thus being less energy-consuming 
can be used for the light box of the invention. For accomplishing a still 
more uniform light distribution on the front side, a diffusion plate can 
be provided also in the light box of the invention. 
Depending on the respective use, also arrangement of a liquid-crystal 
display at the front side of the light box is possible. These display 
means, for obtaining high-contrast representation over the whole display 
area, necessitate uniform backlighting which, as explained above, is 
provided by the light box of the invention. If the light box is to be used 
as a lamp or a working place luminaire, the front side is provided with an 
optical film having the same structure as the optical film of the 
reflector. In this case, the surface structure is arranged on the inner 
surface of the optical film while the V-shaped grooves can extend both 
parallel or at right angles to the light source. Such an arrangement of 
the optical film effects a desired light-orientation. Further, the optical 
film at the front side of the light box has the advantage that the person 
working at said working place is not dazzled by the light beams 
illuminating the working place. Also, the arrangement can be such that the 
surface structure is arranged on the outer surface of the optical film. In 
this case, too, the V-shaped grooves can be oriented transverse or 
parallel to the longitudinal extension of the light source. If the surface 
structure is provided on the outer surface, the light is subjected to a 
focussing effect. 
In an advantageous embodiment of the invention, there is provided a 
mounting means having at least one resilient holding element to be plugged 
onto the light source, the optical film of the reflector having its 
central portion fastened to said resilient holding element. By the 
individual holding elements, the optical film and the light source 
(fluorescent tube) are interconnected and kept at a distance from each 
other. Normally, two or three holding elements are required. Due to the 
resilience of the holding elements, caused by the construction as well as 
by the material of the holding elements, the holding elements can be 
easily plugged onto the fluorescent tube and removed therefrom. The 
holding elements are preferably made from a transparent material, the 
optical film being bonded to the holding element by a transparent plastic 
adhesive. By withdrawing the holding elements from the fluorescent tube, 
the optical film of the reflector can be easily removed, e.g. for being 
replaced by a new one. Thus, the above-described mounting device for 
holding the optical film at the fluorescent tube is advantageous 
particularly with respect to the mounting and maintenance of the light 
box. 
By the above holding element(s), the optical film of the reflector is held 
attached to the fluorescent tube by said holding elements only in the 
central area of the optical film, the remaining area thereof being 
supported on the reflector surface. After the fluorescent tube has been 
mounted in its tube sockets in the light box, the optical film is firmly 
secured on the reflector surface. The construction of the holding elements 
is such that, with the holding element remaining immovable, the 
fluorescent tube can be rotated about the longitudinal axis thereof. 
In a further advantageous embodiment of the invention, the holding element 
is provided with a resilient clamping member surrounding the fluorescent 
tube over an angle of more than 180 degrees, preferably up to 270 degrees. 
This clampingly mountable holding member has formed thereon a radial 
spacing bar. A supporting bar is attached to the free end of said spacing 
bar, the optical film being fastened thereto (by bonding). The supporting 
bar is preferably adapted to the curvature of the reflector in the area 
close to the light source, i.e. in the central portion. Thus, the optical 
film, by being fastened to the supporting bar, is given a curvature 
corresponding to that of the reflector.

FIG. 1 is a perspective view of a light box 10, with the side walls and the 
back-lit front wall being partially broken away to reveal the interior 
construction of light box 10. The light box 10 has a parallelepiped case 
12 consisting of a rear wall 14, four side walls 16,18 arranged at right 
angles to each other, and a front wall 20 opposite to rear wall 14. While 
the rear wall and the side walls of case 12 are made from sheet metal, the 
front wall 20 consists of a light-pervious plastics material and acts as a 
light-scattering diffusion plate. The transparent front wall 20 is the 
back-lit front side 21 of the light box 10. Inside case 12, a reflector 22 
is arranged which, as seen in sectional view through light box 10, is 
arranged as a concavely curved plate having its lowermost portion abutting 
the rear wall 14. The concave reflector surface 24, being directed to 
front wall 20, has its surface painted mat white. In the center of light 
box 10, there is arranged a light source consisting of a fluorescent tube 
26 extending in parallel to the longitudinal side walls 16 of case 12 and 
at a constant distance from side walls 16. (The longitudinal side walls 16 
are the longer walls among the four side walls of case 12.) 
As shown in the Figures, the reflector surface 24 has its central portion 
provided with a film 28 which, as evident from FIG. 2, has an even lower 
surface resting on reflector surface 24 and a structured upper surface 
facing the fluorescent lamp 26. Said film 28 is a so-called optical film 
of transparent material, e.g. polycarbonate or polymethylmethacrylate. The 
surface of the optical film 28 directed towards lamp 26 is provided with a 
plurality of V-shaped grooves 30 extending over the width of the optical 
film 28 and being arranged parallel and in direct abutment to each other. 
Because of the V-shaped section, prisms 32 are formed between neighboring 
grooves 30. The flanks of the V-shaped grooves 30 or respectively the 
flanks of the prisms 32 are perpendicular to each other, extending at an 
angle of 45 degrees to the even surface of the optical film 28. As can be 
seen in the drawings, the V-shaped grooves 30 or respectively the prisms 
32 extend perpendicular to the longitudinal axis of the fluorescent tube 
26, i.e. transversely to the longitudinal dimension of the elongated light 
source of light box 10. By the fact that the optical film is narrower than 
the reflector 22 and is arranged centered thereto, peripheral portions 34 
are generated on the reflector 22. Said peripheral portions 34 are in 
abutment to the two longitudinal side walls 16 of case 12 and are exposed, 
i.e. are not provided with the optical film 28. The two peripheral 
portions 34 are parallel to the fluorescent tube 26 and are mat white. 
As indicated in dotted lines in FIG. 1 and shown in greater detail in FIG. 
3, the optical film 28 is mounted at the fluorescent tube 26 by a mounting 
means. In the presently described embodiment, the mounting means consists 
of three holding elements 36 distributed over the length of the 
fluorescent tube 26 and fabricated from a resilient transparent material. 
The holding elements 36 are provided with a clamping member of C-shaped 
section, enclosing the fluorescent tube 26 over a sector of more than 180 
degrees, preferably up to 270 degrees. The clamping member is a 
cylindrical sleeve 38 which, in one circumfertial portion thereof, has a 
gap 39 extending axially over the length of said sleeve. Through this gap 
39, sleeve 38 is shifted onto the fluorescent tube 26. With the sleeve 38 
being mounted on the fluorescent tube 26, the sleeve 38 is spread, thus 
exerting a clamping force on fluorescent tube 26. In this manner, the 
holding element 36 is secured on the fluorescent tube 26. 
The sleeve 38 has a spacing bar 40 formed thereon, being arranged 
diametrically opposite to gap 39 and extending radially to sleeve 38. The 
radial orientation of said spacing bar 40 determines the distance between 
the optical film 28 and the fluorescent tube 26. The free end of spacing 
bar 40, being averted from sleeve 38, is joined by a supporting bar 42 
which extends transverse to spacing bar 40 and, in the range of the 
fluorescent tube 26, is adapted to the curvature of reflector 22 and has 
the optical film 28 attached thereto by a transparent adhesive 44 (cf. 
FIG. 2). The optical film 28 lies on the reflector surface 24 of the 
reflector and is maintained in its position exclusively by the holding 
elements 36 plugged onto the fluorescent tube 26. The fluorescent tube 26, 
in turn, is mounted within case 12 by the sockets at the ends of tube 26 
(the sockets being omitted in the Figures for reasons of clarity). 
Further, it should be mentioned that, in the Figures, the thickness of the 
optical film and the surface structure thereof are not represented in 
their real dimensions with respect to the other parts of the light box 
since, if the Figures were true to scale, the optical film would not be 
visible anymore. While the light box has a width of about 40 cm, a length 
of 70 cm and a height of about 7 cm, the optical film has a thickness of 
about 0.5 mm, the height of the prisms 32 (or the depth of the grooves 30) 
being about 0.17 mm and the distance of adjacent prisms being about 0.35 
mm. The fluorescent tube 26 is arranged at a distance of about 5 mm to the 
optical film 28; for the better understanding of the invention, also this 
aspect is not shown in correct scale in the Figures. 
Following the above description of the construction of the light box, the 
functional principle thereof will be explained hereunder. 
The front side 21 of light box 10 has placed thereon, e.g. a back-lit 
poster or a photograph arranged on the transparent front wall 20 of case 
12. The front wall 20 is back-lit by the light of the fluorescent tube 26. 
Illumination of the front wall 20 is effected, on the one hand, by light 
beams emitted from the upper portion of fluorescent tube 26 and incident 
directly onto the inner side of front wall 20. On the other hand, however, 
illumination of the front wall 20 is also effected by light emitted from 
the lower portion, i.e. the lower half of fluorescent tube 26, and being 
reflected by the optical film 28 or by the exposed strips of the 
peripheral portions 34 of the reflector towards the front wall 20. The 
light beams impinging on optical film 28 are either reflected by the 
flanks of the V-shaped grooves 30 and prisms 32 or, being refracted, 
penetrate into the optical film 28 and are diffusely reflected by the 
underlying reflector surface 24 of reflector 22 and, after penetrating the 
optical film 28 repeatedly or undergoing multiple reflection within the 
optical film, exit in the direction of front wall 20. In the exposed 
peripheral portions 34 of the reflector area, reflection is diffuse 
because the entire reflector surface 24 is mat white. Due to said diffuse 
reflection in the peripheral portions 34 of reflector 22, a scattering of 
the impinging light occurs in these areas, resulting in a more uniform 
light distribution in the area of the longitudinal edges of front wall 20. 
Moreover, by the V-shaped grooves 30 and prisms 32 on the surface of the 
optical film 28 facing the fluorescent tube 26 in the immediate vicinity 
thereof, it is provided that the light beams are for the largest part 
reflected past the fluorescent tube 26 towards the front wall 20. Other 
than is the case with an even reflector surface, those light beams which 
are reflected in the immediate vicinity of fluorescent tube 26 do not 
penetrate the fluorescent tube 26 and, therefore, do not add to the 
portion of light emitted directly from the fluorescent tube 26 towards the 
front wall. Instead, those light beams which are reflected due to the 
structured shape of the surface of optical film 28 in the area of the 
fluorescent tube 26 contribute to an additional illumination of areas 
immediately near the fluorescent tube 26. As a result, illumination of the 
central area of front wall 20 is more uniform. 
Due to the geometry of the fluorescent tube 26, the light beams exit at 
different distances to the reflector 22 or, respectively to the optical 
film 28. Thus, the light beams impinge at substantially equal angles of 
incidence in wide areas of reflector 22 or, resp. optical film 28 so that 
substantially equal reflections are obtained. Also this effect adds to a 
still more uniform illumination of the front wall 20. In conclusion, it 
should be noted that due to the optical film 28 having V-shaped grooves 30 
and prisms 32 extending transverse to the longitudinal dimension of 
fluorescent tube 26, uniform light distribution is accomplished within the 
box 10 between the reflector 22 and the front wall 20 to be back-lit. 
Therefore, the uniform illumination of the front side of the light box 
need not be "paid for" by a diffusion plate to be arranged in that 
location. As commonly known, diffusion plates cause losses of light, 
necessitating the use of more powerful light sources for obtaining a 
desired luminance in the back-lit front side area. The light box as 
described here and shown in the Figures can be evenly illuminated by a 25 
Watt fluorescent tube without identifiable variations in luminance on the 
front side 21 of light box 10.