Glossary: D

Dark Current

Thermal current produced in an operating photodetector device when no optical radiation impinges on the detector.

  See also Threshold Current

Darkfield Illumination

In microscopy, refers to light that enters a translucent object through the edge perpendicular to the lens. In machine vision, an illumination geometry which has most of the light bounce off of the object; light from edges and surface defects is scattered to the detector yielding a high contrast bright spot. 

Decibel (dB)

A logarithmic measure of relative power levels. It is used to specify the amount of attenuation in optical fibers. 

  See also Attenuation , Optical Fiber

Depth of Field (DOF)

The distance through which an object being imaged may move in or out of the plane of best focus while maintaining an acceptable level of contrast at a particular spatial frequency or resolution. 

Depth of Focus

The image-space equivalent of depth of field. For a fixed object distance, the maximum displacement from the plane of best focus through which the image maintains a particular contrast at a specific spatial frequency. 

Design Wavelength (DWL)

A common specification for optical components describing the specific wavelength of light at which the components are intended to be used, or the wavelength of light used in specifying or testing an optical component for a wavelength dependent specification such as reflectivity or transmission. Optical components used over a range of wavelengths will typically have optimal performance at the DWL and decreasing performance as the wavelength moves further away from the design wavelength. DWL has units of length and is most commonly reported in nanometers (nm) or microns (μm).

 See also: Wavelength.


A measure of the detecting ability or sensitivity of a photodiode. The reciprocal of noise equivalent power (NEP). 

Dichroic Beamsplitter

A type of plate beamsplitter that splits incident light based on wavelength. 

Dichroic Coating

A filter or mirror coating that transmits or reflects light depending on wavelength rather than polarization. The color varies with angle of incidence and thickness of deposition material. Metallic coatings tend to be more spectrally flat. 

Dichroic Filter

A type of filter coated with thinfilms to achieve a desired transmission and reflection percentage across a given spectrum. It is often used as a color filter (both additive and subtractive). A dichroic filter is slightly angle sensitive but is much more forgiving than an interference filter.

  See also Filter

Dielectric Coating

A type of coating consisting of materials that are electrical insulators. The reflective coatings consist of alternating layers of higher and lower index materials (compared to the substrate) in order to achieve a certain reflectivity over a certain wavelength region.


The change in intensity distribution of waves caused by constructive and destructive interference as they contact an obstruction, yielding a distribution that differs from the incident wave-front's. 

  See also Diffraction Grating

Diffraction Grating

An optical component used to disperse light into its individual wavelengths. As incident light strikes a transmission grating's groove spacing, it is dispersed on the opposite side of the grating at a fixed angle. As incident light strikes a reflective grating's groove spacing, it is reflected and dispersed on the same  side of the grating at a fixed angle. 

Diffraction Limit

A theoretical limiting factor governing the maximum obtainable resolution of an optical or imaging system, defined by the inverse of the wavelength of light being used multiplied by the f/# of the lens.

Diffuse Illumination

Diffuse, spatially uniform light from an extended source. Used for even illumination of an object. 


The effect of light scattering over a large solid angle. Light is diffused by reflecting from or transmitting through an irregular (rough) surface. Typically, precautions are taken to remove diffusion in applications; however, diffusers can be utilized to introduce a specific amount of diffusion in order to create a desired effect (i.e. minimizing glare, homogenating a beam, removing speckle in a laser application, etc).

Digital Signal

 A signal that changes in regular steps.  The signal level at each step is represented by a number. 

Digital Signal Processing (DSP)

Found often in high-end industrial cameras, it involves analog-to-digital conversion of all or part of a standard video signal to enhance/change the resulting signal recovered upon digital-to-analog conversion. Enhancements usually pertain to RGB balancing for more accurate color reproduction.

  See also Analog Signal , Digital Signal

DIN (Deutsche Industrie Norm)

An industry standard for the design of microscope objectives and eyepieces. DIN standard microscopes have a 160mm tube length.


Unit of optical measurement of a lens that equals the inverse of the focal length of the lens in meters. 

Directional Illumination

Point source illumination from single or multiple sources.


A measure of how much the index of refraction of a material changes with respect to wavelength. It also determines the separation of wavelengths known as chromatic aberration. Quantitatively, dispersion is inversely given by the Abbe number.


Nonlinear geometrical aberration in which magnification changes with field height (i.e. no distortion at center), defined at maximum field. If imaging a square grid, positive distortion gives a pincushion effect while negative distortion yields a barrel effect. 

  See also Magnification , Aberration

Distortion, Non-Monotonic

Distortion which changes sign, as well as magnitude, across the image. Also called wave or mustache distortion. 


A term that describes the degree to which a light source expands as the distance from the laser increases. Divergence is generally specified as a full angle and can be used to predict spot sizes at a given distance through tangent calculation. Beam divergence can be reduced for long distance uses by expanding the diameter of the beam. 

  See also Fan Angle , Beam Divergence

Double Gauss Design

An imaging lens design that employs low f/# imaging lenses that provide superior overall correction and produce less distortion than standard fixed focal length lenses. However, this design can be subject to residual oblique spherical aberration. Two Gauss lenses, each comprised of a negative doublet and a positive singlet, are situated symmetrically in the design. Double Gauss lenses are commonly used in machine vision applications.

Double-Concave (DCV) Lens

A type of singlet lens with two inward, equally curved surfaces and a negative focal length. Optimized for infinite/ infinite conjugates and ideal for image reduction and to spread light. 

Double-Convex (DCX) Lens

A type of singlet lens with two outward, equally curved surfaces and a positive focal length. Optimized for finite/ finite conjugate imaging and ideal for electronic imaging, relay systems and image projection. 

Doubler Tube

Also known as a doubler, an imaging lens adapter that consists of a negative focal length achromatic lens that doubles the magnification for a specified working distance by giving half the field of view.

Doublet Lens

A lens with two optical components such as an achromatic lens.

Dove Prism

A type of prism that inverts an image by rotating it by twice the prism rotation angle. It is available in two versions: uncoated and coated. An uncoated dove prism is used for image rotation. A coated dove prism retroreflects an image.

  See also Retroreflector

Drive Ratio

In motorized stages, the ratio of motor revolutions per leadscrew revolution.

Dynamic Range

The difference between the lowest detectable light level and the highest detectable light level. Physically, this is determined by the saturation capacity of each pixel, the dark current or dark noise, the ADC circuits, and gain settings. 

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