Is there an iphone app to check if the brightness of your computer monitor is too bright? I've been looking for an app to check if the brightness of my computer monitor is too high and is straining to my eyes.
Does such an app exist that measures brightness of a monitor using the camera perhaps?
I'm not sure this is exactly what you're looking for, they can be pretty expensive, but apps made for professional designers and photographers can do this.  DisplayMate and  Datacolor Spyder5Elite are two often cited products.

On the free side, there's several dozen that use the iPhone camera, including LuxCamera, Luxi, and LightMeter Free, to measure light.  Although, YMMV with these free apps, they should be able to give you a semi-decent reading.

I use a free application named f.lux to automatically adjust the 'blue' light emitted by my monitor.  It works great and I would strongly recommend it.