Part IV: Spectral Distribution of DMD/LED and Camera sensitivity

A project log for Holoscope - Superresolution Holographic Microscope

Subpixel imaging using the Raspberry Pi and an Android Smarthpone. The lightsource is represented by an LCD.

beniroquaibeniroquai 05/21/2016 at 05:230 Comments

In the last post I was talking about the efficiency of the setup. When illuminating the sample with a but-coupled LED one is loosing about 95% of the LEDs light-power. In case of the DMD it’s a bit less, but without the trouble to make the light-guide fitting in the LEDs housing.

The spectral-bandwidth of the in-line microscope should not extend 20nm measured at full width half maximum. Therefore one can use a nodge-filter. To get the highest power from the LED, the spectrum of the LED with smallest bandwidth has to be determined therefore.

In the Graph below, the normalized emission spectrum of the projector (with RGB LEDs) is shown. Peak intensity is at blue-line at 445 nm. Smaller wavelength also gives the opportunity to image smaller details which can later be seen.

To find the video signal, which raises the blue LED, a RGB-ramp is displayed over the VGA-output of a laptop. The following Graph shows the setting for the highest intensity at smallest wavelength-bandwidth. R=0, G=81, B=255; The other „pure“ maxima (Green and Red peak) are way less and decrease the overall efficiency.

After filtering the DMD signal, the spectra looks as follow:

The rainbow in the graph shows a weired behaviour oft he DMD. The way how the DMD-projector shows a specific colour is, that the 3 channels (RGB) are displayed sequentially. The eye is not „recording“ fast enough to see each channel separately, but the camera does. There is a „beat“ ( between the capturing frame rate and the refresh rate oft he DMD.

This can be avoided by using a different LED (disassemble the setup) or put a capacitor in parallel which removes the frequency of the LED-switching voltage.

This setup has the main advantage, that the highest intensity of the system is used and the spectral bandwidth is still quiet small. This follows in less pixel-noise and better results of hologram-reconstruction.