ABSTRACT
As described in Chapter 5, a pixel in the sensor of a digital camera records the incident irradiance integrated over the time it is exposed. This value is called the exposure. If the scene is static, the irradiance is constant and the exposure is simply the product of the irradiance and the amount of time the shutter is open:
X = EΔt. (6.1)
The irradiance E reaching a sensor pixel is affected by a number of factors, such as attenuation in the lens and other imperfections in the optical system. That aside, the sensor pixel does not record the precise exposure value due to physical limitations of the sensor as well as processing by the camera. It is very difficult to model all these processes accurately. Instead, the conversion from ideal scene radiance values to pixel values is modeled as a function of the pixel exposure. This function (or its graph) is known as the response curve of the device (Figure 6.1). The discussion here refers to digital cameras, but the same principles apply to film cameras. The difference is that film does not have discrete pixels. Exposure at a point refers to the irradiance on a small neighborhood of a point on the film. The response curve of a film camera is really the response curve of the particular film with which it is loaded.
