Is there an iphone app to check if the brightness of your computer monitor is too bright?
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.