Light measurement units are defined around very specific spatial responses. A carefully designed input optics system is essential if measurement accuracy is to be achieved.
Two laws of geometrical optics apply directly to radiometry and photometry: the inverse square law and the cosine law.
The inverse square law defines the relationship between irradiance from a point source and the distance to the measurement surface. It states that the intensity per unit area varies in inverse proportion to the square of the distance between the source and surface.
In other words, if you measure 16 W/cm² at 1 meter, you will measure 4 W/cm² at 2 meters, and can calculate the irradiance at any other distance. A well defined measurement plane and approximate point source are required for valid interpolations.
In radiance measurements (W/cm²/sr), where a detector with a narrow viewing angle looks at a much larger, uniform area source, the area of the source that the detector sees increases as the detector moves farther away, counteracting the effects of the inverse square law and making the reading independent of distance.
Radiance measurements are useful for characterizing large area sources such as CRT's and backlit displays, or for diffuse, uniformly illuminated surfaces.
The cosine law refers to the relationship between the irradiance on a surface and the incident angle. The intensity falls off in proportion to the cosine of the reflected angle, since the effective surface area is reduced as the angle increases.
Irradiance and Illuminance detectors, especially those with filters that restrict off-angle light, require cosine correction to properly quantify light coming from all angles. Inaccurate cosine response is one of the biggest sources of error in full immersion applications.
There are many applications in light measurement that require specialized input optics. For example, intense laser beams must be diffused so that the narrow beam does not locally saturate the receiver. Many high intensity ultraviolet sources require temperature resistant remote optics that offer cosine spatial responses and side viewing.