# D

### D*

D* is the detectivity of a detector that indicates the S/N when radiant energy of 1 W is incident on the detector. Since D* is normalized by a photosensitive area of 1 cm2 and noise bandwidth of 1 Hz, it is independent of the size and shape of the photosensitive element. In the upper equation, S is the signal, N is the noise, P is the incident energy in W/cm2, A is the photosensitive area in cm2 and Δf is the noise bandwidth in Hz. The higher D* value, the better the detector.$$D^{*}=\frac{S/N \cdot \sqrt{\triangle f\ }}{P\cdot \sqrt{A\ }}$$ $$D^{*}=\frac{ \sqrt{A\ } }{NEP}$$

### Dark count

This is the number of counts per second that a detector generates in the absence of light.

### Dark current

A small current which flows when a reverse voltage is applied to a photodiode even in a dark state. This current is called the dark current. Noise resulting from dark current becomes dominant when a reverse voltage is applied to photodiodes (PIN photodiodes, etc.).

### Dark resistance

This is the resistance of a photoconductive device in the dark state.

### DBR (distributed Bragg reflector)

This is a reflector containing a diffraction grating having a cycle of λ/2n (λ: wavelength in vacuum, n: refractive index of medium) formed outside the light emission region in light-emitting devices such as LEDs and semiconductor lasers in order to selectively reflect the light of wavelength λ. In VCSEL (vertical cavity surface emitting lasers), forming DBR layers as the upper and lower layers of the light-emitting layer at an appropriate distance causes resonance only at a specific wavelength, so the laser beam can be emitted in the direction perpendicular to the surface. In some LEDs, a DBR layer is formed underneath the light-emitting layer to increase the light level.

### Diffraction grating

An optical element designed to obtain a spectrum by making use of light diffraction. Reflective diffraction gratings usually have a great number of grooves formed in their surfaces and utilize diffraction images created by interference with light beams reflected from the grating surface.

### Double-heterostructure

A structure where a low-band-gap energy semiconductor material is sandwiched between high-band-gap energy semiconductor materials. Since carriers are confined in the low-band-gap region (emission region), the carrier density increases to allow efficient electron-hole recombination.

### Drift - Lamps

Drift refers to variation of output over a long period of time. It can be caused as a result of the change in thermoelectric discharge characteristic of the cathode, change in gas pressure, dirt on the window, or voltage to the lamp. It is expressed in variation per hour.

### Drift - Photomultiplier tubes

While operating a photomultiplier tube continuously over a long period, the anode output current of the photomultiplier tube may vary slightly over time, even though operating conditions have not changed. Among the anode current fluctuations, changes over a relatively short time are called "drift," while changes over long periods such as 103 to 104 hours or more are called the life characteristic. Drift is primarily caused by damage to the last dynode by heavy electron bombardment. Therefore the use of lower anode current is desirable. When stability is of prime importance, keeping the average anode current 100 times below the maximum is recommended.

### Dynamic range

The term dynamic range refers to the range in which a measuring device or detector is capable of accurately measuring the signal. Dynamic range is the ratio of maximum to minimum signal levels (detection limit) that can be detected.
For a CCD the dynamic range is the saturation charge (full well capacity) divided by the readout noise. Since the detection limit depends on both dark shot noise and readout noise, the dynamic range varies with operating conditions, such as operating temperature and charge integration time.
The dynamic range is determined under operating conditions where the CCD is cooled adequately so that dark shot noise can be ignored. Since the upper detection limit is determined by the full well capacity, the dynamic range (DR) is given by:$$DR = \frac{full\ well\ capacity}{readout\ noise}$$or$$20\ log \biggl( \frac{full\ well\ capacity}{readout\ noise} \biggr)\ [dB]$$