Multispectral imaging systems and techniques have become powerful tools for the rapid measurement of high-spatial-resolution spectral images. Liquid crystal tunable filters (LCTF)attached to a monochrome CCD camera are being widely used as spectral analysis systems in different applications, particularly in colour imaging. Among these applications is the measurement of the spectral radiance profile of inhomogeneous sources. In such a system, uncertainty and resolution of the effective spectral distribution of the source mainly depend on that of the system’s responsivity as well as on that of the LCTF bandpass function. In the present work, the bandpass function of a liquid crystal tunable filter has been determined, using both the indirect method and the direct one. It has been demonstrated that the results yielded by either method are not equivalent, which implies that, when the bandpass function needs to be known with low uncertainty, this curve has to be measured by means of the direct method. Not only does the indirect method yield an erroneous bandpass function, but it also provides a different relation between the band’s central wavelength and the nominal wavelength. The effective radiance values that are obtained based on the direct method’s bandpass function can be up to 5% higher or 4% lower than those obtained from the indirect method’s bandpass function. The spectral variation of this discrepancy is oscillating, showing a higher amplitude for the lower wavelengths. This fact can make the measurement errors to be rather large for those radiation sources having narrow spectral distributions, such as lasers or LEDs.

Peer reviewed