33. In earlier cameras where a photocell was used to control the exposure, the photocell could be fooled into giving a reading which did not result in a good picture.

34. This was especially true of flash pictures taken of subjects close to the camera. Light-colored clothing would reflect too much light to the photocell causing it to stop the exposure too soon.

35. On the other hand, dark clothing would fool the photocell into timing out a long exposure, since relatively little light was reflected back. This would result in a washed-out face.

36. To overcome this problem, the Model 636 uses a photocell with a two-part filter in front of it, called a Dynamic Infrared Photometer system. The top half of the filter passes visible light which the human eye can see. This light is used to time exposures for non-flash, fill-flash, and low-light, distant scene flash pictures. The top filter is a photopic filter.

The bottom half of the filter passes infrared light which is invisible to the human eye, but which the film can see. This light is used to time exposures for flash pictures in low light where the subject is near the camera. The bottom filter is called the infrared filter.

37. Now you may be wondering why infrared measurements greatly reduce over- or underexposures in close-up flash pictures.

38. This chart shows the light spectrum. In the infrared region of the spectrum, a strange

phenomenon occurs. The light reflected off just about all materials is equal. Therefore, the light passing through the infrared filter is balanced, preventing the photocell from being fooled by contrasting levels of light or dark subject matter.