If you are using an Optoma Interactive flat panel for HDMI output to push the same content that is currently showing on the IFP screen to a secondary monitor device; in this case a Black magic ATEM switcher. Then you may experience the image to flash or flickering.
Please test a secondary monitor (different brand) to see if you can get standard signal to pass through as a test. If that works please continue below.
It's always good to make sure to test other cables since 90% of the time, flickering problems are cabling related.
If you have already tested good cable and issues persist see following below:
You may achieve success using Blackmagic ATEM input 1 instead of the other input ports of the ATEM switcher. ATEM input 1 has a lot more flexibility to different refresh rates and color settings and has been proven to work for this specific application.