# Lumens per watt

#### LogicalBeard

##### Newly Enlightened
Any help is appreciated; does anyone know how to look at a Cree spec sheet and determine lumens per watt? Specifically the xd16 s3?

#### LEDphile

##### Enlightened
Lumens per watt are just that - the number of lumens out divided by the number of watts in. So to get the lumens/watt number from the datasheet, you need lumens and watts from the datasheet.

Lumens are given in a few places, normally max/min/typical values in a spec table (at a specific current/temperature), a binning range that may narrow down what to expect by CCT/CRI combination, and then correction curves based on the actual drive current / temperature (both of which will affect the efficiency).

Watts are given by the forward voltage of the LED (similar to lumens, there will be nominal values and correction curves) multiplied by the drive current.

#### LogicalBeard

##### Newly Enlightened
Lumens per watt are just that - the number of lumens out divided by the number of watts in. So to get the lumens/watt number from the datasheet, you need lumens and watts from the datasheet.

Lumens are given in a few places, normally max/min/typical values in a spec table (at a specific current/temperature), a binning range that may narrow down what to expect by CCT/CRI combination, and then correction curves based on the actual drive current / temperature (both of which will affect the efficiency).

Watts are given by the forward voltage of the LED (similar to lumens, there will be nominal values and correction curves) multiplied by the drive current.
Thank you so much for the reply. I feel like I might be getting it now. So the data sheet table I first came to for 4000k s3 shows at 350mA and 85C "156" is the minimum lumens. The forward voltage at 350mA and 85c is 2.73V. So 2.73 * .350 is .9555 watt. So basically a watt.

so for that specific temp and amps the lumens per watt is 156 I think.