In this article a mathematical method is established for predicting the monochromatic radiative properties (optical constants) of liquid radiation filters. The liquid radiation filter, LRF, is used as a selective solar radiation absorber for controlling the greenhouse temperature in a hot climate. LRF strongly absorbs the UV (ultraviolet) and NIR (near infrared) wave bands and highly transmits the PAR (photosynthetically active radiation) wave band. Moreover it is used as physical plant growth regulator in photomorphogenesis studies. The monochromatic optical constants of the LRF such as the refractive index nf, λ, the absorption coefficient σf, λ and the dimensionless extinction coefficient kf, λ are mathematically investigated by using the ray tracking technique. The transmittance due to absorption of the LRF τf, λ was investigated by considering the reflectance on the fluid surface. The consideration of a simplified refractive index nf, λ instead of a refractive index with complex part (nf, λ-ikf, λ) for the LRF is also examined because the LRF is highly absorptive radiatively and highly conductive electrically. The results of the present study show that all the monochromatic optical constants of the LRF can be investigated mathematically by using some limiting measured values of the spectral transmittance and reflectance. The simple refractive index nf, λ can be used for the LRF instead of the one with a complex part.