Today’s blog is the first in a series about green lasers, and in particular about how wavelength, power, efficiency, and lumens relate to each other.  Also, I’m also going to write a little about the difference between and R&D announcement and what needs to be known to build a projector.

I believe that direct green lasers are the key to making very small embeddable pico projectors regardless of the display technology be it LCOS, DLP, or Laser Beam Scanning (LBS).   Unfortunately the “physics” of green lasers makes them hard to produce.

The laser makers know that somewhere in the range of 532 nanometers (nm) is a very good wavelength for  “green” in a display.   Some of the key reasons being lumens (brightness), efficiency, and color space (which will be discussed in the next part of this series).

The problem laser makers have is that going from say 510nm to the longer wavelength of 532nm is difficult and they often end up trading off the stability/lifetime/yield=cost; and/or the efficiency; and/or the power output of the laser to get a better wavelength.   Thus when I see a company announce a “breakthrough” with a better wavelength, I’m curious what they had to give up to get it.  So as the laser makers improve the wavelength, they often have to then go back and “fix” things that they “broke” to get the longer wavelength.

To make a marketable projector there are a number of important spec’s we need to know, some of the key ones being :

1. Wavelength
2. Efficiency (electrical power to light power)
3. Power of the light out
4. Lifetime (experimental lasers often degrade quickly)
5. When it will be available and at what cost at what volume

Often the R&D announcement usually talks about 1, 2, or at most 3 of the above specs and almost never #4 and #5 above.     But it is kind of hard to build a product when you don’t know what it will cost and when it will be available.

Sometimes the prices is give in the form of a “riddle wrapped in an enigma” that almost sounds like and answer to #5, such as “the million piece price will be $X.”  But notice it didn’t say how many dollars you will spend on the first 10 and 100 thousand on your way to buying 1 million units or in what year you will be able to buy those million units.

Now on to discuss some of the technical parameters:

Light is a form of energy/power measured in Watts and Lumens or the “photopic response” is how bright a human perceives light.  It turns out that humans have different sensitivities to different wavelengths/colors of light.   At the extreme ends of the light spectrum for example, infrared and ultra violet are invisible to humans and thus produce no “lumens.”

The graph at the top shows how a “Watt” of light of a given wavelength is perceived by human in terms of lumens.   As the graph shows, a 532nm green has 603 lumens/Watt, whereas, 510nm green has only 344 lumens/Watt or about 57% the lumens/Watt of the 532 green.  In between, 525nm green with 542 lumens per Watt or about 89% that of 532nm green.

Something else to notice is that a typical blue laser is around 455nm and only produces about 33 lumens per Watt.  You need the blue to mix with the green and red to get white and the color in between, and it takes power yet produces few lumens.

A typical red laser is in the 640nm range and has 120 lm/W.  This is a deeper “red” than necessary for typical display application and is inefficient, but just like it is difficult to make longer wavelength greens, the physics of the red laser is just that it is difficult to build stable shorter wavelength red such as 625nm.

Wall Plug Efficiency (WPE) – The WPE is the simply the light output in Watts divided by the electrical power (voltage x amps) put into the laser diode.   The WPE of the laser does not include the power of the drive circuitry external to the diode which can be significant.

Note that WPE alone does not factor into it the wavelength.  So if you are comparing a 510nm WPE to a 532nm WPE in terms of lumens per power Watt, you really need to multiply the 510nm green WPE by 57% to compare equivalent lumens per Watt of electrical power.

In the next article in this series on diode green lasers, I’m going to discuss the color space problems with 510nm green (namely it can’t be used to get a good bright yellow).   For those who know about color spaces or can figure it out just from a a diagram, I have included below a CIE chart with the color space triangles for 510nm, 525nm, and 532nm green holding blue at 455nm and red at 640nm.

Credit:  I presented a version of the 2 figures in this post at SID 2011 in conjunction with our paper (co-authors of the paper where Dr. Bill Mei and Dr. Shawn Hurley)