Irradiance is the measurement of the amount of light that shines from Ember's DLP projector through the tray onto the build head. Because this light is the energy that builds your models, it's essential that the projector is calibrated to the proper irradiance value. If this setting goes out of range, it can cause prints to fail.
The calibration process simulates the actual printing process as closely as possible. It is performed with the resin tray in place. This requires the resin tray to be thoroughly cleaned, so as to not damage the light probe.
- Nitrile gloves
- Safety glasses
G&R UV Light Meter (or similar) with a 420nm or 405nm probe
- Ember resin tray
- Optical wipes, e.g., microfiber cloth or lens paper (for cleaning the PDMS)
- Delicate wipers, e.g., Kimwipes (for cleaning the glass surfaces)
- Paper towels (for cleaning the hard plastic of the resin trays)
- Isopropyl alcohol (at least 90%)
- Exposure test file
- Remove the build head from Ember's build arm
- Drain and thoroughly clean the resin tray that you will use for printing
- Clean the two glass surfaces in the optical path
- Install the cleaned resin tray
- Upload the exposure test file to Ember
- Measure the irradiance through the PDMS of the resin tray
- If necessary, modify the "Projector LED Current" in the Printer Settings on emberprinter.com
- Measure the irradiance again, and repeat steps 5 and 6 until the irradiance is the correct value (see Step 7 and 8)
Remove the build head from Ember's build arm. This will provide space to perform the calibration.
NOTE: If you don't remove the build head, it will crush the light probe and break the PDMS of your resin tray, so be sure to remember this step!
Drain the resin tray that you will use for printing.
Above is a photo of the setup that we use in our lab to drain and filter the resin from the resin tray.
Once the tray is drained, inspect its window. If the PDMS is damaged or badly clouded, discard it and replace it with a new tray.
If the PDMS passes your inspection, prepare the resin tray for the luminance calibration process by thoroughly cleaning it. Spray IPA into the tray and wipe it dry with paper towels. Do not wipe the PDMS with paper towels, as this will scratch and damage its surface. Only use optical wipes (e.g., microfiber cloth or lens paper) to wipe the PDMS.
Allow the tray to dry before proceeding to the next step.
NOTE: Because the irradiance calibration is done through the resin tray window, calibration results are tied to the specific tray that you're using. Due to the fact that the UV transmission of the PDMS of the resin tray window decreases over time, the apparent irradiance will decrease over the life of a tray. This can be compensated for by increasing the projector's irradiance in this calibration process, but eventually, you will need to replace your resin tray. When you start using a new tray, be sure to do new irradiance calibration.
There are two glass surfaces in the optical path of the projector. One is the glass plate on the under-side of the resin tray. The other is the glass the protects the projector, beneath the rotating plate. Use Kimwipes moistened with IPA to clean both of these glass surfaces, removing any obstructions in the optical path that could block or diminish the light of the projector.
Install the cleaned resin tray onto Ember's rotating plate.
NOTE: Confirm that the the build head has been removed from the build arm before proceeding to the next step.
Download the exp_test.tar.gz test file and upload it to your Ember. This file exposes Ember's entire build area with the RGB values set to 255 (the maximum value).
Place the probe of the radiometer on the clean PDMS of the resin tray, in the center of the window. If you're using a G&R Labs unit, set one switch to 20 and the other to "MW/CM2." Record the light intensity to one decimal place. As the value may drift over the course of the exposure, take the reading over the approximate duration of an exposure - e.g., 2 seconds.
The baseline irradiance value is 20 mW/cm2 using a G&R Labs Model 221 radiometer, or 25 mW/cm2 with a G&R Labs Model 222 radiometer. Prints will still work if the value is within +/-2 mW/cm2 of the baseline value, so don't worry about getting it exactly perfect. Note that it's better to tend toward too much light rather than not enough.
If the irradiance value is more than 2 mW/cm2 smaller or larger than the baseline value, you will need to manually adjust the value. To do this, log into your account on emberprinter.com, navigate to the Printers page, and click on Printer Settings in the row that corresponds to your printer. In the popup window, click Show Advanced Settings to reveal the Projector LED Current field.
If your printer is not paired with emberprinter.com, you can modify the Projector LED current via SSH.
To modify the irradiance of Ember's projector, enter the appropriate value in this field.
- 255 is the maximum
- 0 is minimum (effectively turning the projector off)
- -1 resets the projector to factory defaults (which could be anything but is usually somewhere around 180)
The process of accessing this field and modifying its value is shown below:
When you have modified the irradiance using the Projector LED Current value, return to Step 6 to retest the value. Repeat Steps 6 and 7 until the baseline irradiance value measures between 18 to 22 mW/cm2 (Model 221) or 24-27 mW/cm2 (Model 222).
An Alternate Approach...
If the measured illuminance is no less than 10mW/cm2, instead of changing the Projector LED Current value as instructed in Step 7, you can compensate for decreased irradiance by increasing the exposure time in the print settings. Use the following formula to calculate the exposure compensation:
Standard exposure time x (desired irradiance / measured irradiance) = New exposure time
For example, the desired irradiance using Model 221 is 20, so if measured irradiance is 11mW/cm2 and the standard exposure time was 1.8 seconds, the new exposure time would be 3.27 seconds:
1.8 seconds x (20.00 / 11) = 3.27 seconds
Be sure to make this exposure compensation for all layers (First Layer, Burn-in Layers, and Model Layers).