Depth of Field

The working range of the beam in or near the focal plane of a lens; a function of wavelength, diameter of the unfocused beam, and focal length of the lens.

Depth of Focus

The distance over which the focused laser spot has a constant diameter and thus constant irradiance.

Dichroic Filter

Filter that allows selective transmission of colors desired wavelengths.

Diffraction

Deviation of part of a beam, determined by the wave nature of radiation and occurring when the radiation passes the edge of an opaque obstacle.

Diffuse Reflection

Takes place when different parts of a beam incident on a surface are reflected over a wide range of angles in accordance with Lambert's law. The intensity will falloff as the inverse of the square of the distance away from the surface and also obey a cosine law of reflection.

Diffuser

An optical device or material that homogenizes the output of light causing a very smooth, scattered, even distribution over the area affected. The intensity will obey Lambert's law.

Divergence

The increase in the diameter of the laser beam with distance from the exit aperture. The value gives the full angle at the point where the laser radiant exposure or irradiance is 1/e or 1/e^{2} of the maximum value, depending upon which criteria is used.

Duty Cycle

Ratio of total "on" duration to total exposure duration for a repetitively pulsed laser.
