The curing of a layer in a 3D printer depends on a lot of different factors ranging from the temperature of the resin to the material used in the printer window. However, the biggest factor is the amount of light passing thru the window. Herein, I'll just refer to this as the "dose" although what matters is the dose per unit area.
For a DLP printer calculating the dose is pretty straightforward:
Dose = light intensity * exposure time
For an SLA printer like the Form 1+, it get's a little more complicated:
spatially averaged dose = Laser Power / Laser Speed / scan line spacing * number of passes
The exact local dose is going to vary, but if the line spacing is appropriate for the laser things should mostly even out.
Because the efficiency of the mirrors and windows are never 100% it pays to check the actual dose of the printer, rather than just assume. This instructable will guide you thru the process.
Step 1: First We Clean
The entire exposed optical path should be cleaned when intensity measurements:
1) The glass side of the tray should be inspected for resin and finger prints. It can be cleaned with acetone and a wiper.
2) The PDMS window should be carefully cleaned with lens or microfiber cloths. This surface is easily damaged.
3) If the mirrors or dirty clean the dust of with compressed gas.
Step 2: Create a Print File
There are two good options for creating a test file. First, one needs to decide what settings file to use.
1) If you are familiar with OpenFL you can create your own settings file and you will know ahead of time what the dose should be. The attached file should in theory give you an output of 50 mJ/cm2. If your output is lower than the value you want, you can just modify another settings file to get the correct dose.
2) If you do not want to use OpenFL, then use whatever resin setting you want to print with most often and measure the output with a brand new (never touched resin) tray. Once the light output drops below some threshold value you should consider getting a new tray (you will just have to figure out from experience with the particular resin). Keep in mind that dose used for each resin may be different, so always use the same material type and thickness when checking the dose.
Once you have a settings file, then just load a block into Preform and send it to the printer (without supports).
Step 3: Measure
One the printer file is created and uploaded to the printer you are almost ready to go. Remove the build head and then position the radiometer probe where desired (the part of the bed you will be printing in). I laser cut a couple of jigs so I can accurately and repeatedly position the probe, but these are entirely optional.
After the laser beam passes over the probe you have the actual output of your printer. The actually output dived by the theoretical output is your efficiency and knowing that it is simple to calculate the desired dose:
desired spatially averaged dose = Laser Power / Laser Speed / scan line spacing * number of passes * efficiency
Step 4: Bonus 2: Light Variation Across the Printer Bed
In order to determine if the dose was uniform across the printer bed I laser cut two jigs to position the G&R labs probe, and then measured the dose at 13 positions across the printer bed. A 124 mm block was used as the test file. It turns out the dose is about 10% lower at the edges relative to the middle. This variation is not bad by any means, but certainly it is another factor too consider when a print fails.