# FAQs | Infrared detectors

### What is the maximum light level allowed to enter an InGaAs PIN photodiode?

The light level at which the output becomes saturated is about 10 mW, although depending on the element size, incident light spot size, and reverse bias. To avoid possible damage to the photodiode, use it at a light level of 1 W/mm2 or less.

### How much does temperature affect InGaAs PIN photodiode noise?

Photodiode noise is expressed by the square root of the sum of the squares of the Johnson noise and the squares of the shotnoise. Johnson noise is dependent on dark current. Dark current is temperature-dependent, so the dark current decreases with a drop in the sensor temperature. Shot noise, however, has no temperature dependence except for special cases (near cutoff wavelength). As the temperature drops, the photodiode noise also tends to decrease. Contact our sales office for formulas for calculating noise.

### What is the difference between photovoltaic detectors and photoconductive detectors?

Photovoltaic detectors generate a current when exposed to light. Photoconductive detectors change their electrical resistance (conductivity) when exposed to light and a bias voltage or current must be applied for use.

### Is chopping required when using photoconductive detectors?

Chopping is not always required. However, chopping is commonly used to improve the S/N by distinguishing signal light from extraneous light in the surrounding environment, etc.

### What is the optimal chopping frequency for photoconductive detectors?

A general guide to chopping frequencies is shown below. The lower chopping frequency depends on the 1/f noise of the detector itself, while the higher chopping frequency is limited by the response speed of the detector. So we recommend using a chopping frequency in the flat region unless a specific problem arises. The chopping frequency we use for testing InSb photoconductive detectors is 1200 Hz.

Product name Chopping frequency
InSb photoconductive detector Several kHz

### How does the photosensitive area of a photoconductive detector relate to detection characteristics?

The signal value is not dependent on the photosensitive area, but the noise is inversely proportional to the square root of the photosensitive area. The S/N is proportional to the square root of the photosensitive area.

### What technique is used for re-evacuating a metal dewar?

To re-evacuate a metal dewar, use the dedicated re-evacuation tool (A3515 valve operator for metal dewar) by connecting it to an exhaust device. We also perform re-evacuation for customers (a service fee is charged).

### Is a dewar usable right away, even after it was filled with liquid nitrogen?

After the white vapor that is generated when liquid nitrogen is injected can no longer be seen, wait for 10 minutes to elapse before using.

### What is the cooling hold time in which a metal dewar is capable of holding nitrogen?

The nitrogen cooling hold time of a typical metal dewar is approximately 20 hours immediately after purchase. The hold time gradually decreases, and after one year it is about 10 hours. So re-evacuation is needed once every few years.

### What sensors are best for measuring objects at different temperatures?

The higher the object temperature, the higher the emission energy, so the wavelength distribution shifts to the short wavelength side. At low temperatures, on the other hand, the energy is small, so the wavelength distribution shifts to the long wavelength side and the energy level at short wavelengths suddenly drops. A general guide to detectors suited for different object temperatures is shown below.

Temperature of object Detector
500 ℃ or higher Si
200 ℃ or higher InGaAs
100 ℃ or higher InAs
50 ℃ or higher InSb, InAsSb (8 μm band)
0 ℃ or higher InAsSb (10 μm band)
-50 ℃ or higher Thermopile, Type-II superlattice infrared detector