Measuring the Light Intensity of the Ember Printer

The first step to successful printing with the Ember printer is measuring the light intensity. One can start printing without checking the light intensity (particularly with the Autodesk CMYKW resins), but your chance of success is much higher if the intensity is within 10% of the correct value. Due to the uptake of optically active resin components the intensity can be much lower than expected, and one has to correct it back 20 mW/cm2.

Note, most of this information is available in the Ember Manual (along with a bunch of other good stuff).

Step 1: First We Clean

The entire exposed optical path should be cleaned when intensity measurements:

1) The exposed printer window tends to get dirty easily. Dust can be removed with compressed gas; uncured resin with acetone and a wiper; and cured resin with a razor blade (very carefully!).

2) The glass side of the tray should be inspected for resin and finger prints. It can be cleaned with acetone and a wiper.

3) The PDMS window should be carefully cleaned with lens or microfiber cloths. This surface is easily damaged.

Step 2: Load the Print File and Measure

The easiest way to get the printer to turn on the whole build areas is just to create a print file that is a 64 mm x 40 mm x whatever block. Rather than just doing one long exposure it is best to keep the exposure time similar to what your exposure time will be while printing.

The reason for this is the output of the LED changes slightly as it warms up from operation. If a 10 or 100 s exposure is used, the output will be much lower than during a print cycle. Conversely, it is also good practice to let the printer run thru a few cycles, so that the LED reaches similar to that during an actual print. The attached file should suffice for most users.

I find keeping a USB drive with the print file already on (in the folder Ember/USB) handy to get users to check the intensity regularly. However, in this case I am loading a file over the network as I am using a test file corrected for he individual printer as detailed here.

Step 3: Measure the Intensity

For the G&R labs radiometer, calibration just involves removing the build head, and pressing print. For other radiometers with thicker probes (ILT 1400) it may be necessary to remove the entire build arm.

If the intensity is between 18 to 22 mW/cm2 you are good to go.* If not go on to the next step.

* All of the intensity data published relating to the Ember printer has been using the G&R labs 420 nm probe. This probe is not perfectly matched to the LED spectrum and gives a value that is approximately 12% low. If you are using a NIST traceable probe that has been recently calibrated for 405 nm you should aim for about 22.5 mW/cm2. G&R labs supplies such a probe and other manufactures can calibrate their probes as needed.

Step 4: The Intensity Is Too Low, Now What

Adjust the intensity

1) Change the LED current value using the advanced printer settings on emberprinter.com (shown above)

2) SSH into the printer. Edit the value of "ProjectorLEDCurrent" in the file /var/smith/config/settings/ and then type: "echo refresh > /tmp/CommandPipe" to update the settings.

Adjust the dose

If you cannot increase the intensity enough or are in a hurry you can just keep the dose per model layer the same by increasing the exposure time. This works most of the time.

The dose is simply,

Dose = Light intensity * time

so the new dose you want to use is

New Dose = (20 mW/cm2) / Light Intensity * Default exposure time.