Abstract:
A handheld, pen-like colorimeter for measuring the color of an object is provided. The colorimeter includes several light emitters, each with distinct color spectra, wherein the emission of each color is modulated at a specific frequency. These light emitters may be Light Emitting Diodes (LEDs) and/or lasers. The colorimeter also contains at least one light sensor which samples light reflected from an object illuminated by the light emitters. The rate of sampling is at least twice the modulation frequency of the emitted light. A microprocessor computes the fourier transform of the intensity of the reflected light over time, wherein the fourier transform provides the light intensity at each possible modulation frequency and determines the relative contribution of the reflected light from each light emitter, as well as the contribution of ambient light. A color value is calculated and a color name is selected and presented to the user.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to co-pending U.S. patent application Ser. No. 09/844,388 entitled “Portable Colorimeter” filed even date herewith. The content of the above mentioned commonly assigned, co-pending U.S. Patent applications are hereby incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to methods and devices for the measurement of the color of reflected and emitted light, and particularly to a handheld calorimeter. 
     2. Description of Related Art 
     Many fields of endeavor require quick accurate measure of the color of objects, or a comparison between objects. In addition, there are many color blind people that have difficulty accurately judging colors to varying degrees. There are also people who may not be color blind, but have simply not learned all of the subtle color variations and names. When these people read books, work on a computer, shop for clothes, etc., they may not always be able to tell the color of the objects at which they are looking. 
     Currently, handheld calorimeters are available for measuring the color of an object. These devices measure color by placing the tip of the probe against (or in close proximity to) the surface of the object being measured. The calorimeter generates a single measurement from three data points representing the reflectance of the three primary colors red, green, and blue (RGB). The single color value can then be compared to a preloaded table of color values. 
     However, current handheld calorimeters have several limitations. Current calorimeters cannot measure color at a distance and have problems handling ambient light. Current methods also have problems with changes in the intensity of artificial lights, such as florescent lights. In addition, the prior art requires recalibration by the user upon every use. 
     Therefore, it would be desirable to have a handheld calorimeter that can measure the color of distant objects and can properly compensate for ambient light, without the need for constant recalibration. 
     SUMMARY OF THE INVENTION 
     The present invention provides a handheld, pen-like colorimeter for measuring the color of an object. The colorimeter comprises several light emitters, each with distinct color spectra, wherein the emission of each color is modulated at a specific frequency. These light emitters may be Light Emitting Diodes (LEDs) and/or lasers. The colorimeter also contains at least one light sensor which samples light reflected from an object illuminated by the light emitters. The rate of sampling is at least twice the modulation frequency of the emitted light. A microprocessor computes the fourier transform of the intensity of the reflected light over time, wherein the fourier transform provides the light intensity at each possible modulation frequency and determines the relative contribution of the reflected light from each light emitter, as well as the contribution of ambient light. The modulation frequency of the light emitters is adjusted to account for the modulation frequency of artificial, ambient light. A color value based on the fourier transform of the reflected light is calculated and then mapped to a list of color values from which a color name is selected and presented to the user. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIGS. 1A and 1B depict schematic diagrams illustrating a handheld, Light Emitting Diode (LED) calorimeter in accordance with the present invention; 
     FIG. 2 depicts a graph illustrating fourier transforms based on modulation frequency in accordance with the present invention; 
     FIGS. 3A and 3B depict schematic diagrams illustrating a handheld, laser calorimeter in accordance with the present invention; and 
     FIGS. 4A and 4B, schematic diagrams illustrating a handheld calorimeter employing both LEDs and lasers are depicted in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is a simple pen-like device used to measure the color of reflected light. The pen calorimeter indicates an object&#39;s exact color on a liquid crystal display (LCD) on the side of the pen (similar to the displays on clock pens). The invention can be implemented in two ways. The invention can also be implemented as a combination of both methods. 
     Referring to FIGS. 1A and 1B, schematic diagrams illustrating a handheld, LED calorimeter are depicted in accordance with the present invention. The tip  101  of calorimeter  100  contains three Light Emitting Diodes (LEDs)  110 - 112 , each of which emits a distinct color of light. In the present example, LEDs  110 - 112  emit red, green, and blue light respectively. Because red, green and blue (RGB) are the primary colors of light, all other colors are a composite of these three colors. Light sensor  113  detects light reflected off the target object. 
     The illumination from LEDs  110 - 112  is modulated with a certain frequency. This permits time-varying detection of reflected LED light and the separation of the LED light from the relatively constant ambient light or ambient light from artificial sources which vary with different frequencies. Such an approach can eliminate the need for white balance detection since the reflective component due to the laser illumination can be separated from the ambient light. 
     Referring to FIG. 2, a graph illustrating fourier transforms based on modulation frequency is depicted in accordance with the present invention. It should be pointed put that the Graph in FIG. 2 is not drawn to scale. Each of the colored LEDs could be modulated at different frequencies permitting simultaneous measurement of their reflective contribution by examining the (time) fourier transform of the reflected light. There would be three peaks corresponding to the three LEDs, each modulated at a unique frequency. The fourier transform would also reveal ambient light sources that are modulated (e.g. fluorescent lights). If artificial lights appear to be modulated at frequencies too close to the frequencies used to modulate the laser sources, the calorimeter can change the modulation frequency of the LEDs to keep them away from the interfering frequencies of the ambient light. 
     To implement the frequency modulation, the reflected light is sampled at a frequency greater than twice the modulating oscillation. A fourier transform of the signal will reveal the reflection magnitude at the frequency corresponding to the LED modulation and thus provide a reading of the color component corresponding to the LED source. The fourier transforms gives direct values for the reflected amplitudes of the different emitters. The x axis represents the modulation frequency and the y axis represents the amplitude. 
     Natural background light will have an amplitude peak N around 0 Hz on the x axis, corresponding to zero frequency modulation (i.e. steady light from the sun). There might also be peaks F 1  and F 2  around 60 Hz and 120 Hz respectively. These peaks represent the modulation frequencies of artificial background lighting, such as that from fluorescent or other forms of artificial light. For red (R), green (G) and blue (B), each color is modulated with a different frequency. For example, the modulations might be 1000 Hz for R, 2500 Hz for G, and 3700 Hz for B. The height of the graph at x=1000 Hz will give the amount of reflected red light, the height at x=2500 Hz indicates reflected green light, and the height at x=3700 Hz represents the amount of blue reflection. The choice of 1000, 2500, and 3700 Hz might not be the best choice under particular circumstances and are only used for the purpose of illustration. The key is to choose modulation frequencies which are not too close to the frequencies of artificial light sources. In addition, the modulation frequencies of R, G, and B should not be integer multiples of each other or any of the other frequencies a user might encounter (i.e. artificial light). By avoiding the integer multiples, harmonic distortions do not influence the measurement. 
     Harmonic distortion from slight imperfections in the circuitry, detector response, or non-linearity of the colorimeter&#39;s light emitters causes additional bumps to occur in the fourier transform at integer multiples of the modulation frequencies. For example, if the modulation for red is 1000 Hz and green is 2000 Hz, some red measurement could bleed over into green. 1000 Hz might not be a good choice in countries that use 50 Hz power lines. However, since 1000 is 20 times 50, the harmonic distortions might not be significant because the 20th harmonic is usually quite weak. The higher the harmonic, the lower the interference. 
     A microprocessor in calorimeter  100  matches the fourier transforms of the reflected LED light to a color name and displays the name on the LCD display  102  on the side of the colorimeter  100 . The color name might also be presented to the user by means of an audio speaker employing Text-to-Speech (TTS) technology. The color names can be stored in a table in internal memory, or in an external source to which the calorimeter  100  is connected. 
     Prior art calorimeters require the user to recalibrate the calorimeter every time the device is used, using either a white or black surface. The present invention, by contrast, allows the user to maintain calibration in memory, thus eliminating the need for constant recalibration with each use. 
     The present invention may also include a focus option to allow the user to average the colors over a larger area in order to measure the general color of a finely colored or patterned area. 
     In addition to RGB, the present invention may also measure colors outside the visual spectrum, such as infrared (IR) and ultraviolet (UV). A calorimeter which includes IR or UV may prove useful in medical fields when trying to assess the health of tissues or in agricultural fields when monitoring the health of plants. Other examples include geological application for determining mineral content, especially at a distance, i.e. a rock wall. 
     Depending on the needs of the client, additional functions may be added to the present invention. One such option is storing measured colors in memory. This would permit the user to annotate the measurement with something as simple as a digit or number, or with something more complex, such as text or a voice clip description. These options may require more powerful microprocessors, extra interface devices (i.e. microphone, keypad, buttons, etc.), and additional memory. However, such additional features and expenses might be reasonable for particular applications (e.g. medical diagnosis or manufacturing quality control). 
     Another additional function that can be added to the present invention is the use of algorithms to coordinate colors of different objects. The algorithms would tell the user which colors could go with other stored colors in particular situations. An obvious application of this function is wardrobe selection, for both professional as well as home use. 
     Additionally, functions could translate the color measurement into something other than a color name. The calorimeter may use the color to identify the object being measured. For example, in a clothing store, each kind of garment may have a different possible set of color choices and thus a certain color measurement may translate into “Cambridge rugby shirt” or other appropriate name. As another example, industrial and commercial users may find it convenient to color code component parts and color code package containers according to delivery priority or destination. 
     Referring to FIGS. 3A and 3B, schematic diagrams illustrating a handheld, laser colorimeter are depicted in accordance with the present invention. FIG. 3B illustrates the front of tip  301  which contains the color-measuring components. The basic design of colorimeter  300  is similar to calorimeter  100 . However, whereas calorimeter  100  uses LEDs  110 - 112 , calorimeter  300  employs RGB laser diodes  310 - 312 . Prior art calorimeters which rely on light emitting diodes (LEDs) cannot measure color at a substantial distance because the LEDs cannot illuminate objects far away. The use of laser diodes in the present invention overcomes this shortcoming because lasers can maintain focused beams over greater distances than prior art, which is why lasers are employed in range finders and targeting systems. 
     In the present example, laser diodes  310 ,  311 , and  312  emit red, green and blue light (or other appropriate colors suitable for color measurement) respectively. The laser diodes  310 - 312  shine on the object one at a time, while a special laser sensing diode  313  measures the reflected light. To facilitate color measurement at long range, light sensor  313  might also include a telephoto lens. 
     A microprocessor inside the body of colorimeter  300  computes the RGB fourier transforms, correcting for sensitivity and calibration. The microprocessor then matches the fourier transforms to a color name and displays the name on the LCD display  302  on the side of the calorimeter  300 , similar to calorimeter  100 . 
     An infrared laser diode can also be added to calorimeter  300  to extend the color spectrum measured. 
     Referring to FIGS. 4A and 4B, schematic diagrams illustrating a handheld calorimeter employing both LEDs and lasers are depicted in accordance with the present invention. This embodiment of the present invention combines the features of the previous two embodiments. The tip  401  of calorimeter  400  contains RGB laser diodes  411 - 413  and light sensor  414  as well as RGB LEDs  415 - 417 . 
     The technology represented by the pen-like devices described above may be applied in other handheld formats. For example, the colorimetry technology of the present invention could add color measurement features to digital still cameras and video cameras or camcorders. The user presses a button and moves a target indicator over a part of a recorded image or viewfinder image to choose an item or area for color measurement. This can be accomplished by using the LCD display and scrolling button that typically appear on these cameras. This process works well at a distance, uses the camera&#39;s existing white balance capabilities, and allows for more options involving memory and processing. Digital cameras typically include powerful microprocessors to compress the images and large storage devices to hold the images. The camera can also be placed in a mode for continuous color measurement of a central or general portion of the current electronic viewfinder. The text description of the color measurement can be superimposed on the viewfinder image as well as optionally on the recorded image, or on a separate display device on the camera. The text can include the raw RGB values as wells as color names and information comparing the measured color(s) to those previously stored for comparison, matching, or other purposes. 
     The color coordination and comparison features described above can also be added to video colorimetry and applied to cinematography. Such an application could be particularly important in light of the recent introduction of digital motion picture cameras. 
     Typically, digital cameras are also sensitive to infrared light and this capability could be better exploited using the present invention to achieve superior results over the current state of the art primarily in medical, or other scientific fields. 
     The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.