In the standard “Step Scan Mode”, each detector data acquisition point correlates to a specific translating roof mirror position. The SPS control software would set the mirror position and acquire the corresponding detector data via the GPIB interface of the lock-in amplifier. Hundreds of positions are scanned in this manner. Since this standard step scan mode can be time consuming, a faster but less accurate rapid scan mode is available as an option. In this “Rapid Scan Mode”, the roof mirror moves continuously and does not stop at each reference position for detector data acquisition. An interferogram is generated immediately after the mirror completes its translation typically in 15~60 seconds. Due to the speed requirement, the modulated source signal using an optical chopper and lock-in amplifier is not used. Instead, the signal from the detector is filtered and amplified, and then feed directly to the computer through an 16-bit AD board.
Electronic Signal Processing Unit
In rapid scan mode, the detector signal is not synchronized with a modulated light source using an optical chopper and lock-in amplifier (amplitude chopper for Michelson mode and phase chopper for Martin-Puplett mode) as the optical chopper is too slow for the data acquisition process. Instead, the detector signal is feed directly into the computer using a 16-bit AD Board. Since the incoming detector signal is weak, it is amplified and filtered using an electronic signal processing unit.
Does it work in Martin-Puplett Polarizing Mode? - Since the rapid scan mode does not need the chopper or lock-in amplifier as they are too slow, it would only work in Martin-Puplett polarizing mode with a fixed polarizer analyzer (which is the same as the polarizing output chopper used in a non-rotating position) placed at the output beam. This fixed analyzer or non-rotating polarizing chopper is an absolute requirement for the Rapid Scan Mode to work in Martin-Puplett Polarizing mode.
Frequently Asked Questions about Rapid Scan Mode and Scan Times on the SPS-300
What is Rapid Scan Mode?
The SPS-300 generates a spectra by taking multiple detector readings as a mirror translates across a known distance. Each mirror position results in a different detector reading. In standard step and scan mode, the mirror stops at a prescribed position for each detector reading. By stopping the mirror at each detector reading, the detector may take a long reading for better signal to noise, and the relationship between the detector reading and mirror position is known. In rapid scan mode, the mirror does not stop translating for the detector. Instead, it keeps moving while the detector captures as many readings as possible along the way. The advantage of rapid scan mode is that there is no dead time between readings due to the stop and go motion of the translating mirror, resulting in faster scans. A typical rapid scan mode takes 15~60 seconds. The disadvantages of rapid scan mode are it only allows for short detector reading times, and a less accurate relationship between detector readings and their corresponding mirror position.
How is Rapid Scan Mode Useful?
Rapid scan mode is useful in making a draft or preview scan across a wide spectral region to determine which region needs closer examination. It does this faster than step and scan mode as accuracy is not a concern. However, if you are always scanning the same kinds of samples, then you will find the rapid scan feature useless as you already know which part of the spectrum to look for.
What if we only want to get a spectrum within a relatively small spectral range, e.g. 0.3-0.5THz (10cm^-1 to 16.7cm^-1)? What happens to the required scanning time and the spectral resolution? Do they remain always the same or depend on the size of the spectral window?
This is a very good question in understanding how an FTIR works. In general, the resolution is set by the traversing length of mirror stage and the spectral region is set by the spacing density of the mirror positions within that length (data-point density). The base SPS-300 has a stage length of 50mm which allows it to achieve 0.119cm^-1 resolution regardless where in the spectral region. If we want to scan a larger spectral region, we would scan more points along this 50mm stage length resulting in a higher data-point density. If we want to scan a smaller spectral region, we would scan less points along this 50mm stage length resulting in a lower data-point density. The total travel distance of 50mm remains the same in both cases assuming we want the highest resolution of 0.119cm^-1. Please note that regardless of desired spectral region, we always start from 0cm^-1 and scan upwards to the selected cut off frequency.
For the spectral region 0.3-0.5THz (10cm^-1-16.7cm^-1), we would typically scan the spectral range 0-20cm^-1 as a rounded setting. For this spectral region, we need a data point density of 1.6points/mm. Hence if we want the highest resolution, we would scan at this data-point density for 50mm resulting in 80 detector readings. If we want half the resolution, we would scan at this data-point density for 25mm resulting in 40 detector readings.
In regular step and scan mode, each scan point takes about 300ms. The bolometer detector integration time is about 100ms (for scanning thin films or gases) and the dwell time which is defined as the time between scan points is about 200ms. This 200ms is required for the mirror to move and settle before taking the next detector reading. Hence an 80 point scan would take about 24 seconds in the highest resolution mode. If the high resolution stage is used which would result in a 30cm mirror travel, 480 data-points is required for this spectral region which would require 144 seconds to scan.
What limits scan time and does using Rapid Scan mode help?
Detector sensitivity in capturing a good signal is what truly limits scan speed, and much more so than the difference between using step & scan mode or rapid scan mode. After all, rapid scan mode only eliminates the dead stage translation time between scan data points which becomes irrelevant when the detector integration time becomes the dominant factor. Hence in situations with long bolometer integration time, rapid scan mode essentially degrades down to standard step and scan mode. Using the IR Lab bolometer as the detector and the internal 75W Hg light inside the SPS-300 as the source, the typical bolometer integration time is about 100ms. However, the dwell time, or time between scans, is about 200ms. Hence in typical step and scan mode, the scan time is about 300ms x the number data-points required. In rapid scan mode using the same IR Lab bolometer as the detector and the internal 75W Hg light inside the SPS-300 as the source, the mirror speed is limited to about 0.5cm/s due to detector signal to noise issues. Any faster scan speeds (though mechanically possible), would not yield enough signal for a decent detector reading. This also assumes we are scanning very thin samples or gases. If the material is thick or has low transmission coefficient, the fastest stage speed in rapid scan mode may even be longer. At this stage speed of 0.5cm/s, a scan at the highest resolution would take 10 seconds to complete. Up to 100 detector readings can be obtained at these speeds resulting in a data-point density of 2points/mm.
Technical Specifications

• Uses separate AD Board Interface instead of Lock-in Amplifier to keep up with fast data acquisition
• Includes a signal amplifier to boost pyro-electric/bolometer signal before digitization
• supports stage scan speed of 0.5cm/s (10 seconds on standard 5cm stage). Speed limited by detector S/N