A detector in a typical photo-research system quantifies the radiant intensity applied for inducing photo-response in a sample, or the radiant intensity generated by the sample when it responds to light or other simulation. The measurement is normally performed subsequent to splitting of the beam into its component wavelengths. This is followed by conversion of light signals into electrical signals, which are then amplified, and displayed by signal-to-noise improving electronics. The combination of detection and measurement systems is termed as a radiometer. A spectroradiometer measures radiant intensity versus wavelength information.
Oriel detector systems can be generally classified into thermal detectors and photon detectors. These are further classified on the basis of response generating processes. Wavelength and temporal response modifying phosphorescent accessories are employed in some systems to adjust them to a specific UV measurement situation. Oriel detector systems generally have actual detector elements and suitable cooling, bias, and signal processing electronics to function properly. They also come with software options.
Thermal detectors involve the conversion of the incident radiation into a temperature rise, which is measurable in different ways. Oriel detector systems work by using a sensitive element to measure the temperature change caused by the pyroelectric effect, or the voltage induced at the junction of dissimilar metals.
The sensitive element is blackened, to improve the absorption of the radiation, using a blackening material, which delivers high, and nearly homogeneous absorption, detector responsivity over an extensive spectral range. This is the key benefit of thermal detectors. Controlling the black absorber thickness is necessary to prevent high thermal mass to some detectors, as it hinders the responses time and increases the noise equivalent power.
Types of Thermal Detectors
The thermal detectors offered by Oriel Instruments are of two types:
- Thermopile Detectors for DC radiation
- Pyroelectric Detectors for pulsed, chopped or modulated radiation
Thermocouples are the basis of radiation detecting thermopiles, and consist of two serially connected dissimilar metals. The radiation is detected through absorption, by blackening one junction while shielding the other junction. The temperature gradient at the junction of the dissimilar metals induces a voltage. This phenomenon, discovered by Seebeck, is the basic operating principle of all thermocouple temperature sensors. The principle of operation of a thermocouple detector is depicted in Figure 1.
Figure 1. Principle of operation of a thermocouple detector.
The output voltage can be increased by serially connecting several thermocouple junctions, so that all of the ‘hot’ junctions are placed closely together for collecting the radiation. Such a configuration is called a thermopile, which shows no licker, 1/f, noise as it does not require current bias for operation. Highly sensitive measurements can be performed, from DC to the few Hz frequency response limits of a specific device.
Thermopiles exhibit high sensitivity in the infrared, due to their broadband absorption. Hence, their field of view needs to be carefully stabilized, due to IR emission by all near-room temperature objects, including people. Effective measurements can be made by shuttering the incident radiation on the detector, and monitoring the change in output voltage. The schematic drawing of a thermopile detector is illustrated in Figure 2.
Figure 2. Schematic drawing of a thermopile detector.
A pyroelectric material is typically crystalline, and exhibits electric polarization even when there is no applied voltage. This electric polarization changes when the material is expanded due to heating, building up a charge on opposite surfaces. This leads to a current flow in the circuit connecting the surfaces. Since the current is generated by the temperature range, pyroelectric detectors are sensitive only to pulsed or modulated radiation.
Pyroelectric detectors have a faster responsiveness to changes in radiation when compared to thermocouples or thermopiles. Furthermore, they are not influenced by steady background radiation. Small detectors with low thermal mass respond very rapidly. Blackening the surface to provide homogenous absorption over an extensive spectral range reduces the frequency response, and increases the mass. The schematic drawing of a pyroelectric detector is shown in Figure 3.
Figure 3. Schematic drawing of a pyroelectric detector.
Oriel pyroelectric detectors are intended for, and in, two different modes:
- As detectors of modulated or chopped radiation to generate an AC output signal
- As detectors of isolated or relatively infrequent (<4000Hz) radiation pulses, which may vary in energy from nanojoules to joules, and in width from picoseconds to milliseconds
The thermal and electrical time constants of pulse measuring detectors are selected for integration of each pulse. The peak output voltage is a measure of the charge generated by the detector and, consequently, of the pulse energy. The charge dissipates prior to the onset of the next pulse. The relatively long integration time, or fall time, limits the minimum interval between pulses, or the repetition rate of pulses, which can be quantified individually.
The chopped mode of modulated radiation detectors are often employed in FT-IR and lock-in detection type applications. The pyroelectric detector in a FT-IR instrument monitors the modulated signal, and generates the interferogram, which is then converted into a spectrum data. In a lock-in configuration, the duty cycle of the measured radiation is 50% and the output signal is the average of many chopping cycles. A lock-in detection system virtually eliminates the influence of the background radiation on readings, and its normal 100-200Hz level response frequency limit can be extended electronically to the 1kHz level for fast chopping.
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.