I have some problems with the luminous data of several lamps makers companies. This table resume the situation: http://pacorossofoto.files.wordpress.com/2013/08/par641.jpg (Commas are used in the way it is used in Europe, as decimal separator, not as thousands, so 11,00 is 11.00 not 11 000). The first column "grados" is the plane anle of emmision in degrees. (From catalogue) Second column is the solid angle, calculated from w = 2 * PI * (1-cos (a/2)) where PI is 3.141599, w is the solid anle in stereorradians and a is the plane angle. Third column "candela" is the light intensity in candles (data from catalogue). (This lamps are PAR 64). Fourth column "lumen" is light flux in lumen. It is calculated multiplying the light intensity (3th column) for the solid angle (2nd column) because the light intensity is J = F / w Where F is the flux, J is the intensity and w is the solid angle. This data (flux) is calculated from catalogue data. Fifth column "W" is the electric power in watts (from catalogue). Sixth column "R" is the luminous efficacy calculated dividing the lumen (fourth column) over the power (fifth column). R = F / w. It is, R is lumen per watts. Seventh column is the lamp. OSR stands for Osram, Phil stands for Phillips. The lamps are PAR64 conceived for stage lighting and photographic studio (in wide sense: cinema, TV, etc). My problem... See the R number. About 11lm/w. In the bibliography it is said the typical R for tungsten is about 25lm/w... ¿What do you think about it? (If we talk about flash lamps (strobes, not lanterns) the R is as low as 14) I think I am wrong but do not know where I fail. (Another day we'll talk about why the real BCPS is much lower than the catalogue says).