In short
- Five questions decide the detector: sensitivity, wavelength, optical power, responsivity and the application, from measuring a quantity of light down to counting single photons.
- Most detectors use the photoelectric effect. An absorbed photon frees an electron, and the electrons are measured as a current.
- Each family has its job: photodiodes for most intensity measurements, avalanche photodiodes and photomultipliers for weak light, charge-coupled arrays for images, photoresistors for low-cost light switching.
- Check the signal before you buy. The calculator below turns optical power into photocurrent, output voltage and signal-to-noise ratio.
Choosing an optical detector is not easy: there are many electro-optical options. The right one depends on the sensitivity you need, the wavelength and power of the light, the detector’s responsivity and the application, which can range from measuring a quantity of light to targeted readings of single photons.
The main detector categories are:
- PIN photodiodes
- Charge-coupled devices (CCDs)
- Avalanche photodiodes (APDs)
- Photomultipliers
- Photoresistors
- Photoconductors
Most of these devices rely on the photoelectric effect. A photon whose energy is above the material’s activation threshold (its band gap or work function) is absorbed and frees an electron. The freed electrons can then be measured as a current.
Photodiodes
A photodiode is a p-n or p-i-n junction operated under reverse bias. It produces a current proportional to the light falling on its area. It is often paired with a transimpedance amplifier, which converts the current into a proportional output voltage. Photodiodes are used in position and motion sensing, photography, medical detection and communications.
Standard UV–NIR photodiodes are very versatile. They respond from about 300 nm to 1100 nm, from ultraviolet to near-infrared, and suit both digital and analog signal detection. Photodiodes made from other semiconductor materials extend the range further into the infrared: NIR photodiodes beyond 1100 nm, SWIR photodiodes for the short-wave infrared and MWIR photodiodes for the mid-wave infrared.
Fiber-optic communications typically use PIN or avalanche photodiodes, often in local-area networks. Avalanche photodiodes are preferred where sensitivity matters most, such as low-power lasers or long-haul fiber links, because their internal gain amplifies weak optical signals considerably. For the fiber side, see A Beginner’s Guide to Understanding Fiber Optics.
Photoresistors
A photoresistor’s resistance decreases as the intensity of the incident light increases. Photoresistors are inexpensive, so they are common in light meters and in street lamps, where they switch the lighting on at dusk and off at dawn.
Charge-coupled devices
Charge-coupled devices capture images. Each pixel acts as an independent potential well that fills with electrons when light falls on it (photocapacitance). Typical modules have 5 to 10 million pixels, and the array dimensions affect how fast the data can be moved into memory.
Image sensors saturate at very low power. For high-power coherent sources such as lasers, place neutral-density (ND) filters or a beam sampler between the source and the sensor. This is exactly how a camera-based laser beam profiler measures a laser beam’s shape safely.
Other detector types
A vacuum tube in which a photocathode emits electrons when light strikes it, and a chain of dynodes multiplies them by secondary emission. The output can be millions of times the input, with very low noise and very high gain. Used in astronomy and medicine.
In a phototransistor, light falling on a photodiode junction drives the transistor in place of an electrical input. Light activates the transistor, which is the basis of optical couplers that isolate circuits from electrical sources.
Early developments in nanotechnology, aimed at cell imaging and quantum computing. They are not yet mainstream in optics.
10 tips: match the detector to the application
Use this cheat sheet to quickly identify which detector fits your need:
| # | Application | Optical detector |
|---|---|---|
| 1 | Electrical isolation | Optical transistor |
| 2 | Low optical power | Photomultiplier or avalanche photodiode |
| 3 | Intensity measurements with a voltage output | Photodiode with a transimpedance amplifier |
| 4 | Broad UV to NIR detection | UV–NIR photodiode (300–1100 nm) |
| 5 | Precise UV to NIR detection, short pulses | Fast UV–NIR photodiode (300–1100 nm) |
| 6 | Fiber-optic communications | Photodiode |
| 7 | Fiber-optic communications, long haul | Avalanche photodiode |
| 8 | High power in the NIR band | Large-area NIR photodiode |
| 9 | Higher accuracy in the NIR and IR bands | SWIR or MWIR photodiode, often cooled |
| 10 | Lowest noise, optimum gain | Photomultiplier (PMT) |
Photodiode signal and noise calculator
Enter the optical power on the detector, the wavelength and the quantum efficiency, then the feedback resistor and bandwidth of the transimpedance amplifier. The tool returns responsivity, photocurrent, output voltage and the signal-to-noise ratio.
Responsivity R = η·q·λ/(h·c) ≈ η·λ[nm]/1240 A/W. Photocurrent I = R·P; output V = I·Rf. Shot noise = √(2q(I + Idark)B); feedback-resistor thermal noise = √(4kTB/Rf) at T = 295 K. SNR = I / √(shot² + thermal²). Amplifier voltage noise and 1/f noise are not included, so treat the SNR as an upper limit.
From detector to calibrated measurement
A detector becomes a measurement instrument once it is calibrated in the right units and band. For the radiometric background, read The Basics of Light Measurement: Radiometry and Photometry and the deeper review in Optical Sensors in Radiometry. For ready instruments, see the IZAK power meter module, the multi-channel spectro-radiometer and the laser beam profiler family.
IZAK products for this application
Instruments from our catalog that match the measurements and set-ups in this article.
Optical powerMulti-Channel Power MeterParallel optical power on several fiber-coupled channels, with logging software.View product →
RadiometryMulti-Channel Spectro-RadiometerSpectral and radiometric measurement on several channels at once.View product →
Laser metrologyLaser Beam ProfilersUV to LWIR families, packaged, OEM or software, with Test mode and reports.View product →Need something that is not in the catalog? We design custom electro-optical systems →
Frequently asked questions
QHow do I choose an optical detector?
QWhat is the difference between a PIN photodiode and an avalanche photodiode?
QHow do I calculate photodiode responsivity?
Tzachi Sabati
CEO, IZAK Scientific
Physicist specializing in photonics and quantum technologies, with deep expertise in quantum sensors and advanced optical systems. Leads the Advanced Quantum Lab course at the Technion, bridging academic excellence with industry innovation. At IZAK Scientific, provides cutting-edge photonics-based solutions, developing customized inspection and sensing systems for R&D and production. Passionate about advancing quantum sensing applications and integrating novel technologies to meet industry needs.
