The constantly improving technology of detectors and wide selection of materials and detection methods allows most light detection needs to be satisfied by fairly simple systems. Also with such a wide range of choices, at the same time, with so many choices, selecting the best system can be confusing. This article helps in simplifying the selection process and helps making an educated choice.
Minimizing Impact of Noise
The influence of noise can be minimized by following the below guidelines. Figure 1 shows the four criteria for choosing the best match of detector response to signal characteristics. These sketches will look the same if the abscissas are labeled:
- Field of View
This figure shows a variety of poor matches, a good match and the "best" match.
Figure 1. Examples of poor and good matches of detector responsivity to signal
The wavelength range of the signal must be covered by the detector sensitivity. If the detector's sensitivity wavelength range exceeds that of the signal, any radiation outside of the signal range will contribute to noise. A bandpass filter is used to narrow the range of wavelengths seen by the detector. If the detector and signal ranges miss each other partially, then some of the signal information is lost. The "Good" and "Best" cases in Figure 1 demonstrate full capture of signal information with wavelength mismatch contributing little or nothing to system noise.
The most often adjusted factor in trying to reduce noise contribution is frequency bandwidth. This is because it very explicitly enters the noise equations in the form of
This applies to Johnson and shot noises. Another form of bandwidth control is to move up in frequency to minimize the 1/f noise. AC detection methods, to be discussed in more detail on the next page, take advantage of various forms of electronic and digital filters to match the detection bandwidth to the frequency spectrum representing the rate of change of the useful signal. AC methods are particularly beneficial in the infrared part of the spectrum due to the large amount of background radiation.
Specific events may occur at specific time intervals and for specific lengths of time. There are a number of techniques to first create this periodic behavior and then using it to improve the signal to noise ratio. One must note that the characteristic signal frequency be as different as possible from the natural modulation frequencies of the noise sources (60 or 50 Hz line frequencies and their harmonics are notorious). Some "good" modulation frequencies include 30 Hz (25 Hz), 90 Hz (75 Hz), etc. Higher modulation frequencies are required for measuring fast changing signals. Gated averagers or integrators are used to improve the signal to noise ratio in the measurement of pulsed sources.
Field of View
This is a very important criterion for infrared detectors. Since room temperature objects emit infrared photons, particularly in the vicinity of 10µm, the field of view needs to be narrowed to receive radiation mostly from the source of interest. Imaging and aperturing must be used as this effort will pay big dividends in providing you with meaningful and reproducible results.
General Noise Reduction Techniques
All detectors and signal conditioning electronics have some temperature dependence in their noise and responsivity characteristics. It is possible to obtain more reproducible results if the thermal environment of the experiment is stabilized. The detectivity limits are typically improved by cooling the detector to operate below room temperature.
No source is perfectly stable. Whenever possible, ratio the response signal to that of the source to obtain the most accurate results.
It may sound funny to use AC techniques to measure DC signals but it actually is the best way in practice. The signal can be encoded with a known modulation and then use this characteristic modulation to discriminate against noise contribution which will have its power spread over a different and wider band of frequencies.
An AC coupled amplifier with a narrow band filter centered on the modulation frequency will help in significantly increasing the signal to noise ratio. The filter frequency pass band can be narrowed only to the limit of the modulator stability.
Gated Integration/Boxcar Averaging
These techniques assume their name from the appearance of the pictorial representation of the process, Figure 2. Gates, boxes, or "windows" are used to define the times during which the electronics acquire signal. The signal to noise ratio is improved during those gated times since noise contributions which would be accumulated during the off times are absent. The process, when repeated for N pulses, will lead to signal to noise ratio improvement of N1/2 if the noise is of the white variety, Johnson or shot.
Figure 2. Repetitive signal and detection "windows"
AC coupled detectors may be subject to DC saturation (from background) and thus loss of linearity. You must understand your detector's DC response limits when using it in an AC coupled mode of operation.
It is not possible to beat Johnson noise but its contribution can be minimized to the S/N ratio. It is important to try to make the bias current shot noise, the dominant AC noise contributor in a PMT based detection system. Make this noise three or more times bigger than Johnson noise and you have effectively negated the influence of Johnson noise on the system's S/N behavior In low light situations, begin by selecting transimpedance gain to be the maximum allowed by bandwidth requirements. Next increase raise the bias voltage, within the safe range of the tube, until the dark current AC shot noise is about three times greater than Johnson noise that is variance must be about four times the level of zero bias case.
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.