p.e. (photon equivalent)

This represents the detection level per photon. A 1 p.e. pulse, for example, is equivalent to the pulse obtained when one photon is detected.

Passive alignment

Precise mechanical positioning for coupling alignment between two or more optical elements during optical module assembly. Since the positioning of optical elements usually requires accuracy ranging from submicrons to several microns, highly precise metalization patterns and V-grooves formed by semiconductor process technology are utilized as the positioning reference. In contrast to passive alignment, active alignment performs the positioning of optical modules or fibers while making the optical modules emit light by simulating actual operation (for instance, operating a laser diode to emit light) and monitoring the emitted light in order to obtain the required characteristics of the optical modules.

Phosphor screen

A scintillator in the form of a thin sheet. This is usually fabricated by depositing scintillator material on a support substrate and covering it with protective film.

Photocathode material

Cathode type

The photocathode is a photoemissive surface usually consisting of alkali metals with very low work functions. The photocathode materials most commonly used in photomultiplier tubes are as follows:

1.Ag-O-Cs. The transmission-mode photocathode using this material is designated S-1 and sensitive from the range of visible light to infrared radiation (300 nm to 1000 nm). The reflection mode covers a slightly narrower range from 300 nm to 1100 nm. Since Ag-O-Cs has comparatively high thermionic dark emission, photomultiplier tubes of this photocathode material are chiefly used for detection in the infrared region with the photocathode cooled.
2.GaAs GaAs activated in cesium is also used as a photocathode. The spectral response of this photocathode material usually covers a wider spectral response range than multialkali, from ultraviolet to 930 nm, which is comparatively flat over the range between 300 nm and 850 nm.
3.InGaAs.This photocathode material has greater extended sensitivity in the infrared range than GaAs. Moreover, in the range between 900 nm and 1000 nm, InGaAs has a much higher S/N ratio than Ag-O-Cs. Some photocathodes can operate at 1700 nm.
4.Sb-Cs. Sb-Cs has a spectral response in the ultraviolet to visible range and is mainly used in reflection-mode photocathodes.
5. Bialkali (Sb-Rb-Cs, Sb-K-Cs). These materials have a spectral response range similar to the Sb-Cs photocathode, but have higher sensitivity and lower dark current than Sb-Cs. They also have a blue sensitivity index matching the scintillation flashes of NaI scintillators and so are frequently used for radiation measurement using scintillation counting.
6. High temperature bialkali or low noise bialkali (Na-K-Sb). This is particularly useful at higher operating temperatures since it can withstand up to 175 °C. At room temperatures, this photocathode operates with very low dark current, making it ideal for use in photon counting applications.
7. Multialkali (Na-K-Sb-Cs). The multialkali photocathode has a high, wide spectral response from the ultraviolet to near infrared region. It is widely used for broad-band spectrophotometers and photon counting applications. The long wavelength response can be extended to 930 nm by a special photocathode activation processing.
8.Cs-Te, Cs-I. These materials are sensitive to vacuum UV and UV rays but not to visible light and are therefore referred to as solar blind. Cs-Te is quite insensitive to wavelengths longer than 320 nm, and Cs-I to those longer than 200 nm.

Photoconductive detector

A photosensor which increases its electric conductivity when illuminated with light. An external power supply is needed to operate a photoconductive detector. Photoconductive detectors include MCT (HgCdTe), PbS, PbSe, etc.

Photoelectric effect

A phenomenon in which a substance absorbs light and generates free electrons.

Photon counting

Types of Photocathode

PMTs have extremely high gain low noise amplifiers. The mechanism is capable of amplifying a single electron into a detectable signal. Usually these single electron pulses are summed and read as a DC current. However it is possible to count the single electron pulses; since each pulse represents a photon this method is known as photon counting. It offers some advantages in noise and stability and the convenience of digital detection.

Photon detection efficiency (PDE)

This is a measure of what percent of the incident photons were detected. PDE is expressed by the following equation. The avalanche probability (Pa) becomes larger as the reverse voltage is increased. $$PDE = QE \cdot fg \cdot Pa$$

  • QE: quantum efficiency
  • fg: geometric factor
  • Pa: avalanche probability

Photoresponse nonuniformity

Each of the photodiodes arrayed in an image sensor or CCD is carefully fabricated to provide uniform performance, but each also exhibits some small nonuniformity in terms of sensitivity. This may be due to crystal flaws in the silicon substrate, variations in the wafer process and diffusion in the manufacturing process. This nonuniformity is often called the photoresponse nonuniformity.

Photosensitive area

The area in a device that actually collects light and converts it to electrons. In our data photosensitive areas which are circular are noted with one dimension and quadrilateral squares with two dimensions. Some PMT dimensions are described as 2π, square or hexagonal.


The ratio of photocurrent expressed in amperes (A) or output voltage expressed in volts (V) to the incident light level expressed in watts (W). Photosensitivity is represented as an absolute sensitivity (A/W or V/W) or as a relative sensitivity (%) to the peak wavelength sensitivity normalized to 100. We usually define the spectral response range as the range in which the relative sensitivity is higher than 5% or 10% of the peak sensitivity.

Photovoltaic detector

A semiconductor photosensor generating an electrical current or voltage when light is illuminated on its PN junction. It is capable of operating without power supplied from an external source. Photovoltaic detectors include Si, InGaAs, GaAsP, GaAs, InAs, InSb, etc.

Position detection error

When a light spot irradiates onto a PSD and the resulting current extracted from each output terminal of the PSD is equal, the incident position of the light spot is called the electrical center of the PSD. By considering this electrical center as the origin, the position detection error is defined as the difference between the position at which the light spot is actually incident on the PSD and the position calculated from the photocurrents. We measure the position detection error under the following conditions:

  • Light source: λ=890 nm
  • Light spot size: Φ200 µm
  • Photocurrent: 10 µA

Position resolution (ΔR)

The minimum detectable displacement of a light spot incident on the photosensitive surface of a PSD, expressed as a distance on the photosensitive surface of the PSD. This position resolution is determined by the S/N, which is calculated by: PSD resistance length x noise/signal. We define the position resolution calculated based on root-mean-square (rms) noise measured under the following conditions:

  • Interelectrode resistance: See characteristic table in our datasheets.
  • Photocurrent: 1 µA
  • Frequency bandwidth: 1 kHz
  • Equivalent input voltage noise of circuit: 1 µV

Power dissipation

The maximum power consumption allowed for a device, calculated from the upper temperature limit of the package and chip. In most cases, this is determined by heat-vulnerable components included in the device. Using a coefficient called “derating” makes it possible to calculate the absolute maximum rating for the power dissipation at the temperature at which the device will actually be used. For example, if a power dissipation of 500 mW is defined as the absolute maximum rating at 25 °C and the derating is 5 mW/°C, then the absolute maximum rating at 85 °C will be: 500 mW - 5 mW/°C x (85 °C - 25 °C) = 200 mW.

Propagation delay time

The time required for a signal to travel from the transmitted point to the received point. This term generally indicates the total delay time in the circuit and optical elements, and the delay time in a medium (optical fiber, etc.). The propagation delay time that changes is termed the jitter or wander. Jitter is the fluctuation in propagation delay time that occurs in a short period of time and mainly results from noise. Wander is a long-term fluctuation chiefly caused by thermal factors.

PSD (position sensitive detector)

A position sensitive detector (PSD) consists of a monolithic PIN photodiode with a uniform resistance in one or two dimensions. PSDs have many advantages, compared to discrete element detectors, including high position resolution, fast response speed and simple operating circuits. Position data is independent of the size of light spot on the detector. They can be used for non-contact distance measurement, laser beam alignment and optical tracking of an object. PSDs for tracking electrons or high energy particles are also available.

Hamamatsu makes the following:

1D PSD: Monolithic Si detector using surface resistance to provide continuous position information in one dimension.
2D PSD: Monolithic Si detector using surface resistance to provide continuous position information in two dimensions. Certain photomultipliers can also be used to provide two-dimensional position information.
Special types: A number of products can serve the same function as a PSD, but do not fit into the same general category. These include discrete PSD arrays, devices that can switch from being a photodiode to a PSD, non-linear output PSDs and PSD angle sensors. Signal processing circuits. Various types of signal processing circuits are available for the tracking of a light spot independent of incident light level. Boards are available for one or two-dimensional and either pin-cushion, tetra-lateral or duo-lateral PSDs. The signal processing boards are a compact size and easy to use, no complicated adjustments required. Measurements can be made by simply connecting the PSD and a +15V power supply.

Pseudo-random pattern

An encoded string that is seemingly irregular (not truly irregular) and is used to measure bit error rates and eye patterns. In commonly used pseudo-random patterns, the event probability between 0 and 1 is equal so that a pseudo-random pattern can be relatively easily generated by a circuit that uses a shift register and feedback.