Relative spectral response

The conversion of radiated power (P in watts) to luminous flux (F in lumens) is achieved by considering the variation with wavelength of the human eye s photopie response. Then the spectral power from the source (PA in, lor example, W/nnt) is convoluted with the relative spectral response of the eye (V tabulated by the CIE) according to ... 

Figure 1. Relative spectral responses of different broadband detectors in the UV spectral region. The dashed line corresponds to the CIE action spectrum. The numbers in the legend correspond to the weighted integral (Warm 2) of a standard solar spectrum (30° SZA, 330 DU).

From its principle of operation, a broadband detector provides a signal proportional to the integral of solar radiation 1(1), weighted by its relative spectral responsivity w(X), over its entire sensitivity range. Therefore, the eiythemal dose rate E given in units of W ffm 2 can be than given from (1) ... 

Fig. 6.53. Comparison of the relative spectral response of 0.35 pm thick a-Si H p-i-n cells deposited on glass substrates covered with LP-CVD ZnO and SnC>2. Reprinted with permission from [74]...

Fig. 9. Human eye daytime (photopic) relative spectral response (right axis) and solar spectrum (jagged curve).

Relative spectral responsivity sfij See action spectrum. 

Figure 7-27 shows the relative spectral response of the various kinds of transducers that arc useful for UV, visible, and IR spectroscopy. The ordinate function is inversely related to the noise of the transducers and directly related to the square root of its surface area. Note that the relative sensitivity of the thermal transducers (curves H and /) is independent of wavelength but significantly lower than the sensitivity of photoelectric transducers. On the other hand, photon transducers are often far from ideal with respect to constant response versus wavelength. 

We will deal with the integral of (2.15) by either an in-band approach, or by using relative spectral response data to determine an irradiance or exitance that is effective at a specific wavelength. The first method allows us to calculate the average in-band responsivity of the detector the second yields the responsivity at a specific wavelength. 

In-Band Method We mwif use this method if we do not know the relative spectral response of the detector. We can use this method if we want to know the average responsivity in a given spectral band. 

S.2 Effective Exitance at a Specie Wavelength, and Absolute Responsivity at a Specific Wavelength We do not normally know the absolute spectral response of a detector, but spectrometer tests can often provide the relative spectral responsivity of the detector Jl lA). Given 7i (A), the spectral exitance of the source, and the transmittance of any optical components in the path, we can calculate an equivalent monochromatic exitance (or irradiance) - the effective exitance (or irradiance) at some specific wavelength. We can combine this effective value with the measured broadband signal to determine the absolute responsivity at any desired wavelength. 

The six panels of Figure 4.2 compare the relative spectral response of a thermal detector, a band-to-band photon detector , and a photon detector that depends on transitions between two specific levels - both per photon and per watt. In each... 

In other cases, we are required to determine the responsivity at a specific wavelength - for example, 3.65 pm. That can be done with broadband data, but it requires that we know the relative spectral response, and some mathematical manipulation - see Section 2.2.52. An easier way is to use a narrow-band spectral filter - 3.55-3.75 pm, for example. 

Spectrometers were described briefly in Section 9.3.6. Direct determination of the absolute responsivity of a UUT, K i X), at any specified wavelength is not possible since we do not know with any accuracy the irradiance from the monochromator. Instead, we determine the relative spectral responsivity of our UUT by comparing the signal from our UUT with that from a standard detector whose relative spectral response is known. 

Hubbs (1998) Method to Validate Relative Spectral Response Curves by J. E. Hubbs, J. P. Garcia, and E. L. Dereniak in Infrared Detectors and Focal Plane Arrays volume 3379 SPIE Infrared Detectors and Focal Plane Arrays V, 510 (July 1998) doi 10.1117/12.317620. This is an interesting and instructive analysis of measured data. 

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