The first cut design of a PCB to control the 50W LED by digital current control is complete.  Making Gerbers now.  8-bit control gives 256 levels of brightness from the LED.

It is important when making lights for biological systems to use CURRENT CONTROL ... not voltage/PWM.  An LED controled by a PWM signal only APPEARS to brighten or dim.  It's not really ... it's just fast blinking all on/all off .  The strobing effect is undesirable by lots of plants and animals.  On the AVERAGE, the PWM signal looks analog, but it's really just strobing digital.

I arrange eight mosfets in parallel - each controlling 1/2 the current of its most-significant neighbor, and twice the crrent of its least-significant neighbor.  The most current is controlled by the most-significant bit in the control word/byte. 

To control the 50W LED, the most-significant MOSFET turns 25W on or off (b7).  Then next MSb turns 12.5 W on/off (b6), then 6.25 W, then 3.125 W ... etc etc.

control bit               power controlled

7                                    25 W

6                                    12.5 W

5                                     6.25 W

4                                     3.125 W

3                                     1.5625 W

2                                     781 mW

1                                      390 mW

0                                     195 mW

Voltage is a constant 36 VDC, regardless of current.

Power level on each MOSFET is controlled by a current limiting resistor on the drain.  The sources of ALL MOSFETS are tied straight to the anode of the 50 W LED.

50W/36V=1.4 Amps

36V / 1.4 Amps = 27.5 Ohms resistor on the most-significant MOSFET (a high power beastie, I put two 55 Ohm 12Watt R's there in parallel)

Number 2 MOSFET requires 55 ohms

Number 3 MOSFET:  110 ohms

#4:  220 ...

... you see the trend, I think