The present disclosure concerns a plethysmograph. The plethysmograph comprises a housing defining a test cavity configured to enclose a test subject. The plethysmograph further comprises an optical filter providing a spectrally restricted optical access to the test cavity from an exterior of the housing, the optical filter being configured to at least partially transmit light in a transmission band ranging from about 560 nm to about 750 nm; and to at least partially block light in a blocking band ranging from about 380 nm to about 560 nm.