What is the reflectivity of the mirrors on the DMD DLP5500 at a wavelength of 1550 nm?
Thanks for posting this question. I just looked at the Datasheet titled, "DLP 0.55 XGA Series 450 DMD (Rev. B)," and it has some great information in regards to the DLP5500 (window transmission, and micromirror reflectivity, etc.). According to the datasheet, The DLP5500 is designed for Visible light ranging from 420nm-700nm. It also says the Single Pass Window Transmission (Efficiency) is ~97% and the micromirror reflectivity is around 88%.
So when I reference the Single Pass DMD Window Transmission chart, it shows that around the 450nm mark (For VIS wavelength), it starts approaching to reach towards that 97% efficiency mark. It continues to have that same efficiency of around 95-97% until it starts to hit the 700-750nm range. So from that range of ~450nm to 700nm, the micromirror reflectivity stays around 85-88%.
If you're using a VIS wavelength of 1550nm (we do not recommend you go over the suggested maximum in regards to wavelength. A wavelenfth of 1550nm is twice as much than the recommended wavelength max of 750nm. This could result in stuck pixels on the DMD over time and could eventually cause the DMD to slowly lose performance), the Window Transmission chart shows you are at about 70-75% efficient, so a decrease of about 20% from the recommended range (420nm-700nm). If your micromirror reflectivity during the 95-97% efficiency range is around 88%, I would conclude that the it would drop by at least 25% once it hits that 1550nm range that you're using. So I would estimate the micromirror reflectivity at 1550nm would be around 60% or so, max.
It's hard to say exactly what the micromirror reflectivity would be at this range becuase it is twice the recommended amount and we would not suggest this. But based on the datasheet and the Single Pass DMD Window Transmission charts, this is what I conclude.
Here is the Extranet link to the datasheet I referenced. I believe you will find it very helpful.
I hope this information helps Ben. Good luck!
Welcome to the DLP section of the TI-E2E community. The window transmission chart that Justin is referring to can be found here: http://focus.ti.com/pdfs/dlpdmd/Type_A_window_transmission_Discovery_Website.pdf. This chart is for a different DMD type, but the windows have the same coating and the wndow glass has the same effective optical properties.
The mirror reflectivity follows roughly the reflectivity of bulk aluminum, although other factors make the actual reflectivity slight lower. A chart of Al reflectivity can be found at: http://en.wikipedia.org/wiki/File:Image-Metal-reflectance.png
As you can see from the chart, after a brief dip at 730 nm the mirror reflectivity itsleft at 1000 - 5000 nm is quite good. However from the window transmission chart (first link) you can see that the window transmission drops to about 75% at 1550 nm - this is for a single pass. The result is a total loss of about 45% loss for the cumulative inbound pass plus outbound pass through the glass. The chart shows an IR coated window, but this window is not available for S450 package DMD's.
Justin is correct that TI does not recommend operating a DMD outside of parameters provided in the data sheet, which is 10 mW/cm2 for wavelengths greater than 700nm. However, stuck pixels are not really expected in the IR range.
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