Photographic-Lighting
Color-Balance Technique
Photographers need the camera's color balance, or film type, to match the ambient lighting. Ambient light can vary widely, whether cool daylight, indoors under warm-colored incandescent lights, or under fluorescent lights.
This is a technique developed by Steve Hines for determining the color balance of scene illumination, independent of the color of the subject or scene. For example, the photographer can shoot a subject on a green grassy field, or a red blanket. The camera will ignore the color of the subject. The color balance will be determined solely by the color of the illumination.
For background...
When driving at night after a rain, the reflections in the water puddles on the street are the colors of the mercury-vapor or yellow sodium street lights, the moon, car head lights, tail lights, neon signs, etc. Most of these reflections are partially or fully polarized and easy to analyze. Reflections off most objects (skin, hair, grass, water, wood, floors, cars, glass, plastic, paint, pavement, etc.) are partially or even highly polarized.
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Example:
For illustration, a still-life scene is compared at left viewed normally, and at right through a rotating polarizing filter. For all three pictures shown here, consider that you are looking at the scene, not at pictures taken by the camera.
Normal white-light photograph which contains the colors of the objects, and the reflections of the light sources.
Scene viewed through a rotating polarizing filter which transmits the colors of the objects, and selectively transmits or blocks the polarized reflections of light sources.
The "difference" signal between the two images above, eliminates the color of the scene but leaves the reflections from various mixed illuminants, to be analyzed for color balance.
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:
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•The most economical version samples the scene through a fixed polarizer (like the filter in polarized driving glasses), and compares it to the scene with no filter.
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•A more advanced version compares the scene through two fixed filters, one each with horizontal and vertical axes.
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•The most accurate version, illustrated above, uses a single polarizing filter rotated mechanically or electronically, and compares the difference signals for orthogonal axes. Example at 0° & 90°, at 10° & 100°, at 20° & 110°, etc. continuously through the cycle until parallel to the starting position at 180° & 270°. The preferred technique for rotating the polarizing filter is to position a fixed polarizing filter behind a liquid crystal fed with a saw-tooth signal to twist the axis electronically.
The color analysis does not have to be done using image sensors, as implied with images of the scene here, but with simple red, green and blue photodetectors behind the polarizing filters.
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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 by email or at 818-507-5812.