D

Depth of field

The distance between the nearest and furthest parts of an object that are acceptably sharp while the lens is in focus for a specific distance.

Depth of focus

The range of image distances over which the system can deliver a sharp image (as defined by the user), e.g., the distance that photographic film or printing paper can be moved and still maintain an acceptably sharp focus.

Dichroic filter

Filter that allows selective transmission of desired wavelengths while reflecting others.

Diffraction

The deviation of a wavefront of light from the path predicted by geometric optics when the wavefront interacts with (or is restricted by) a physical object such as an aperture or an edge. Secondary wavefronts develop from this interference, and these wavefronts not only interact with the primary wavefront, but they will interfere with one another to form various diffractive patterns.

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 fall-off 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. (See diffuse reflection.)

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/e² of the maximum value, depending on which criteria is used.

Dosimetry

Measurement of the power, energy, irradiance, or radiant exposure of light delivered to tissue.

Drift

All undesirable variations in output (either amplitude or frequency).

Duty factor

Also known as duty cycle, duty factor (Df) is the ratio of pulse duration to pulse period.