This article describes detection systems. In a typical photo-research system, a detector measures the radiant intensity used to evoke photo-response in a sample or the radiant intensity produced by the sample in response to light or other simulation. The measurement is very frequently done after the beam has been separated into its component wavelengths. Oriel detector systems can be roughly divided into:
- Thermal detectors
- Photon detectors
Most detectors are described by certain figures of merit.
The responsivity S is defined by S = Y/X
Y = detector output
X = radiation input
Examples of Y include current or counts from a photomultiplier or voltage from a thermopile. Y usually has a value Y0 (dark signal) for X = 0. In this case:
X can be any radiative input. The units if X are normally watts or lumens. Where the irradiance on the detector is uniform, X may be given as the product of the irradiance and detector area.
dY(λ) = the fraction of output due to input dX(λ) at λ
Quantum Efficiency (η)
This is the ratio of the number of basic signal elements produced by the detector to the number of incident photons. The basic signal element is frequently a photoelectron.
τ is the time it takes a detector's output to rise when suddenly subjected to constant irradiance. When the irradiance is turned off, the detector output falls to 1/e of the initial value in one time constant.
This is the range of input radiant flux over which the signal output is a linear function of the input. The range is often stated in terms of input watts or input irradiance. This is shown in Figure 1.
Figure 1. Linearity of response of a silicon photodiode.
Noise Equivalent Power
NEP is the radiant flux in watts necessary to give an output signal equal to the r.m.s. noise output from the detector. The flux may be either continuous or sinusoidally modulated.
Normalized Detectivity (D*)
For most detectors DAD1/2 is constant implying that detectivity inversely varies with the square root of the area of the detector. This is because the electrical noise power is usually proportional to the detector area AD, and current or voltage, which provide a measure of that noise, are proportional to the square root of power.
D* is defined to allow comparison of different types of detectors independent of the detector area and bandwidth.
The units of D* are cm Hz1/2 W-1. Figure 2 shows the D* of a number of our detectors.
Figure 2. Approximate D* values as a function of wavelength for some detector types represented in this catalog.
There are several types of noise that limits the detctivity.They are:
- Shot noise
- Generation-recombination noise
- Johnson noise
- Flicker or 1/f noise
- Readout noise for array detectors
Figure 3. System noise.
Noise is seen in photo-conductors in which the absorbed photons produce both positive and negative charge carriers.
Shot Noise is due to the discrete nature of radiation, which is composed of photons arriving randomly in time.
Johnson Noise or thermal noise is caused by the random motion of carriers in a conductor. The result is fluctuations in the detector's internal resistance, or in any resistance in series with the detector's terminals.
Readout Noise is a characteristic of array detectors and is associated with the uncertainties introduced during the transfer of charges between storage registers.
When absolute measurements are required to quantify the light flux, calibrated instruments are needed. Oriel Instruments' Calibration Laboratory has acquired a number of National Institute of Standards and Technology (NIST) source and detector calibration standards, as well as additional standards traceable to international standards' setting and disseminating bodies. Calibration transfer standards, lamps and detectors is typically traceable to the national Institute of Standards and Technology (NIST) to ensure meaningful and reproducible calibrations.
Thermal detectors work by converting the incident radiation into a temperature rise. The temperature change can be measured in several ways.
Two types of Thermal Detectors are:
- Thermopile Detectors for DC radiation.
- Pyroelectric Detectors for pulsed, chopped or modulated radiation.
Radiation detecting thermopiles are based on thermocouples. A thermocouple consists of two dissimilar metals connected in series. One junction is blackened to absorb the radiation to detet radiation. The temperature rise of the junction (with respect to another non-irradiated junction) generates a voltage. This effect, discovered by Seebeck, is the basis of all thermocouple temperature sensors.
Figure 4. Principle of operation of a thermocouple detector. The dissimilar metals 1 and 2 are often bismuth and antimony. The sensing junction usually has a thin blackened metal foil attached to absorb more radiation. The other junction is shielded.
By connecting a number of the thermocouple junctions (typically 20 to 120) in series the output voltage can be increased. All the "hot" junctions are placed close together to collect the radiation. This constitutes a thermopile.
Figure 5. Schematic drawing of a thermopile detector.
The response of a pyroelectric detector depends on two time constants. The thermal time constant determined largely by the thermal mass and the thermal connections from the element to its surroundings, and the electrical time constant which is the product of the shunt resistance and shunt capacitance of the detector/amplifier circuit.
Figure 6. Schematic drawing of a pyroelectric detector.
Photon or Quantum detectors respond to the incoming photon flux in a quantized manner. A relative responsivity curve for a 100% efficient photon detector is shown in Figure 7.
Figure 7. Relative spectral responsivities of perfect detectors.
Figure 8 demonstrates a typical responsivity curve of a silicon detector and the corresponding quantum efficiency curve.
Figure 8. Silicon responsivity and quantum efficiency.
Figure 9. Relative spectral responsivities of perfect detectors.
Photoemissive and Semiconductor Junction Detectors
Photoemissive and semiconductor detectors have much in common.
- The direct photo-electron interaction is very rapid so these detectors have the potential of following fast changing radiation levels.
- Detectivity is in general higher than that of thermal detectors, but over a limited wavelength range.
Photomultiplier Tubes, PMTs
In a photoemissive detector, light interacts directly with the electrons in the detector material. An absorbed photon frees an electron and the surplus energy gets converted into kinetic energy of an electron.
There are several ways of classifying semiconductor detectors. All are photoconductive, but here we use the photoconductive detectors' name for devices which rely on photoconductivity in the bulk and do not necessarily have junctions.
Silicon photodiodes are the most common detectors of light used in instrumentation. The typical structure of a silicon photodiode includes photons passsing through the thin top layer to generate electrons and holes near the junction.
Unbiased Operation - Photovoltaic Mode
Because the 1/f noise increases with bias, this mode has the better NEP at low frequencies. In fact the signal to noise ratio is better than that for biased operation at frequencies below 100 kHz and much better below 1 kHz.
Biased Operation - Photoconductive Mode
Operation with reverse bias reduces junction capacitance and thus increases the speed of response of the diode. It is the preferred mode of operation for pulsed detectors.
Silicon diodes require a low impedance load resistor to provide a linear output in a photovoltaic mode. This makes the maximum output voltage too low to achieve wide dynamic range when using an oscilloscope as a monitor. A transimpedance amplifier solves the major linearity restrictions, by providing near "zero" load impedance, and produces higher output voltages. It also limits the high frequency response of the system.
Multichannel detectors include photodiode arrays and charge coupled devices. The major differences between them lie in sizes of sensitive elements or pixels, the linear versus two dimensional value of the arrays, and the signal handling and readout methods.
Photodiode Arrays (PDAs)
The PDAs are of a linear format. The aspect ratio of individual pixels, 25 pm wide by 25 mm high for the silicon based models, and 50 pm wide by 200 pm high for the InGaAs models, is designed to complement spectrograph function.
In photoconductive detectors, absorbed incident photons produce free charge carriers. These change the electrical conductivity of the detector. An applied voltage, or bias, causes a current to flow which is proportional to the photon irradiance (it helps if the dark resistance is very high).
About Oriel Instruments
Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range from UV to IR, pulsed or continuous, and low to high power.
Oriel also offers monochromators and spectrographs as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.
This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.
For more information on this source, please visit Oriel Instruments.