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I have come across two (apparently) different equations for this purpose. One is mentioned in BASINS software code here

$$Rad\_len [inday^{-1}] = \frac{SolRad[langleysday^{-1}]}{((597.3 - 0.57 * temp[centigrade])*2.54)} $$

while the equation mentioned in FAO-56 Allen et al 1998 (2) in chapter 3 is

$$Rad\_len[mmday^{-1}] = \frac{SolRad[MJm^{-2}day{-1}]}{2.45}$$

I don't see any resemblence between these two equations even after converting $langleysday^{-1}$ to $MJm^{-2}day{-1}$ (which can be done by using a factor of 0.041868). The first equation is used in the evapotranspiration estimation method from radiation using given by in Jensen and Haise, 1963 (2).

(1) Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. Fao, Rome, 300(9), D05109.

(2) Jensen, M. E., & Haise, H. R. (1963). Estimating evapotranspiration from solar radiation. Proceedings of the American Society of Civil Engineers, Journal of the Irrigation and Drainage Division, 89, 15-41

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