Responsivity and Spectral Response
The FTIR provides the spectral shape of the detector's response and the Mikron M305 blackbody source can be used to find the magnitude of the response.
Light from an FTIR excites carriers in a sample, and the amplified output as a function of FTIR mirror position provides your interferogram--take the Fourier transform and you have signal as a function of incident wavelength. Note that the FTIR computer doesn’t know what gain the preamp is using, so if you want to compare spectra collected with different gain settings you should multiply each by their respective preamp sensitivities.
However, you haven’t accounted for the spectral shape of the incident light so you can’t differentiate between wavelengths where your detector responds strongly and wavelengths where a lot of light reaches the detector.
We normalize out the system response by dividing the output spectrum by the spectral response of a pyroelectric detector (keep track of the range where this detector is actually spectrally flat). Typically we use either the Gentec’s spectrum collected in step scan mode or a DTGS spectrum collected via rapid scan then calibrated by fitting to the transfer function. This measurement only needs repeated if something in the setup changes--say you changed out a window. Otherwise, someone in the group should be able to give you the most recently collected system response spectrum.
Here is the latest DTGS calibrated system response (wavenumber vs. response)
date: 11/18/2025 GLaTGS detector from inside 1.808 v80 FTIR
After normalizing out the spectral response setup’s lineshape we are ready to put meaningful values along our y-axis. To do so, we measure the current collected under illumination by a well calibrated blackbody source then calculate the power reaching the detector.
The spectral shape of the blackbody emission very closely follows the ideal blackbody curve and can be easily calculated.
Like with the spectral response measurement, every component in the optical path affects the spectrum reaching the detector--but in this case we need to keep track of how the magnitude changes as well as the shape. Additionally, we care most about the responsivity close to the bandedge and usually don’t put much effort into measuring the shape of the detector’s short-wavelength cut-off. Therefore, we use a bandpass filter to remove blackbody emission that’s much shorter (and longer) than the detector’s design wavelength. This should be the only strongly dispersive element used--the cryostat window and chopper should essentially be spectrally flat. Technically, atmosphere is also dispersive, so it’s best to choose a bandpass in a region where atmospheric absorption is minimal. Additionally, using a filter that covers a wavelength range where your detector’s response is relatively flat can somewhat simplify the following calculation.
So the incident spectral intensity as a function of wavelength is given by the product of the blackbody emission spectrum, solid angle subtended by the detector, and the transmission spectra of each element in the optical path. Then the total power reaching the detector is the product of the area under that curve and the detector area. The current you measured under illumination is what your detector produces for this incident power, so multiply your incident spectral intensity curve by the detector’s spectral response then find the area under this new curve. Using the same domain as for the last integral (usually the range of wavelengths where the bandpass transmission is nonzero), find the area under the arb. unit spectral response curve. If you performed that operation on your actual (correct) responsivity curve then you would get a value equal to the measured current, so scale your entire spectral response curve by the ratio of those two integrals.
Here is a sample MATLAB code to calculate Responsivity. Note, you’d need to change a lot of file paths and some intial set up configuration variables, such as distance between the aperture and the detector, the temperatures, pins, etc.
Tips:
Align without the bandpass filter because the signal will be much stronger. To make this easy we typically have the bandpass mounted in a filter wheel that has an empty slot.
Make sure to connect the Preamplifier into Multimeter to record operating voltages.
If you are not getting a proper signal, double check the connection to the chopper. It often gets loose and even though the proper frequency is reflected on the lock in, it might be the case that the chopper is actually rotating slower.
Make sure frequency is Sine Trig on Lock In
Repeat the measurement with multiple bandpass filters and blackbody temperatures
To minimize uncertainty, it is best to choose a bandpass filter that transmits over a range where the detector’s spectral response is strong, free of atmospheric effects, and relatively flat. Generally, using a spectrally narrow bandpass filter makes this easier.