Photographic-Lighting Color Balance
Photographic-Lighting Color Balance
This camera color-filtration technique provides the correct filtration, based on the color of the lighting, regardless of the color of the subject. A subject can be photographed on green background, or red background, with the same correct color balance of the subject.
When driving at night after a rain, water reflects car headlights, tail lights, neon signs, street lights, the moonlight, etc. Reflections off these puddles and many other materials (glass, plastic, paint, grass, wood, floors, cars, pavement, etc.) are partially or even highly polarized and are easy to analyze.
Example:
To photograph this still-life scene the camera needs to ignore the subject colors and color balance for the illuminant only.
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| The color negative of the normal white-light photograph above contains the colors of the objects, and the reflections of the light sources. | The scene viewed through a rotating polarizing filter transmits the colors of the objects, and selectively transmits or blocks the polarized reflections of light sources. |
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| When added together, the “difference image” eliminates the colors of the objects but leaves the reflections from various light sources that can be analyzed for color. |
This technique compares the direct view of the scene to another view through a polarizing filter. The camera electronics creates a “difference” signal (basically subtracting one image from the other) which negates the colors of the objects, revealing only the colors of the reflections. The subject colors are canceled, and only the reflected illumination, not blocked by the polarizing filter, remains. The circuitry then adjusts the camera for the average color of scene illumination for the best overall color balance.
The technique can be designed into cameras in various ways to favor accuracy vs. cost. Possible versions:
- The most economical version samples the scene through a fixed polarizer (like the filter in polarized sunglasses), and compares it to the scene with no filter.
- A more advanced version compares the scene through two fixed filters, one each with horizontal and vertical axes.
- The most accurate version, illustrated above, uses a polarizing filter (rotated mechanically or electronically), and compares the difference signals to the unfiltered image.
The color analysis does not have to be done using image sensors, as implied with images of the scene here, but with photo-detectors behind the red, green and blue polarizing filters.
This technique is not being offered for license by HinesLab but is shown as an example of technology that consulting clients can expect on a contract basis when engaging the services of HinesLab. Please contact Steve Hines: