I'm hoping to clear up a bit of confusion of how single chip DLP projectors work with lithography. Single chip DLP projectors use a color wheel along with a mercury lamp and a single DMD.
http://upload.wikimedia.org/wikipedia/c ... _wheel.gifThe mercury lamp is on the left in the above graphic. The lamp radiates light through the color wheel as it spins. After light pases through the color wheel it hits a mirror as shown in the following photo.
http://upload.wikimedia.org/wikipedia/c ... t_Path.jpgIn the photo above you can see the path of light after it goes through the color wheel located on the right ot the photo. The mercury lamp is located outside of the frame farther to the right of the color wheel. The light radiates from the lamp and goes through the color wheel. The red arrow shows the direction the light takes after passing through the color wheel. The light is directed at a mirror that reflects the light into the DMD device. The DMD device is made of thousands of micro mirrors that either reflect the light out through the lens or away from the lens into a heat sink. Each mirror represents one or more pixels in the projected image. When the light gets reflected by the DMD out through the lens is creates a bright pixel on the screen or resin in the case of a printer.. When the light gets reflected into the heat sink the pixel is very dark or black.
The spectrum of light radiated by the mercury lamp is very wide it extends from the UV through the visible and into the IR. The total spectra is shown in the chart below.
http://www.olympusmicro.com/primer/imag ... burner.jpgThe color wheel uses three narrow bandpass color filters to only pass Red (635nm), Green (530nm) and Blue (460nm) portions of the total spectra. The filters aren't perfect so a little of the light outside each color still passes through.
Most resins available for hobby SLA are only sensitive to the spectra below 420nm. Some are only sensitive below 390nm (UV). Since the color wheel filters out most of the spectra radiated by the mercury lamp, very little of the spectra actually gets to the resin that is required to trigger the resins to polymerize or cure.
If the color wheel is removed then much more of the spectra emitted by the mercury lamp gets passed through the projector onto the resin. The only spectra that triggers polymerization in these resins is the spectra from 420nm (Violet) and lower (UV) or 390nm and lower for the other resins.
Resins that are sensitive from UV and up into the visible spectrum will polymerize or cure without the removal of the color wheel since they are triggered by all the light that passes through the Blue filter of the color wheel and even some of the Green filter.
http://bucktownpolymers.com/zve200-v470.html is an example of one of these resins that cure from UV past the Blue spectra to over 500nm.
LCD projectors operate differently. For a detailed description of how LCD projectors operate and interact with resins please see this thread:
http://tinyurl.com/l2lpsrz