GregAtkinson I would not recommend the LiFli for displays. The reason is that it needs a pretty high brightness to operate. When the brightness is too low, the scale would just blink at 2%, indicating a lack of light. It would still output to an oscilloscope, but the output is very noisy, which is a problem since this hides low brightness fluctuations. The device works well for measuring room lighting and lamps, which I believe it is made for. But not for low brightness light sources like displays. On displays I need to show a white screen on a high brightness setting to get enough light.
For display measurements, you are much better off buying some BPW34 off eBay and build the oscilloscope circuit. It works even without soldering as you can see in the latest picture.
If you want to see even tiny fluctuations (1%) in low brightness modes, I can recommend the PicoScope 2204a. Its lowest range of +/-50 mV is a true 8 bit hardware mode. Plus it can do oversampling to simulate additional bits of vertical resolution, meaning it can filter out some noise. Up to 10 bits are useful. They told me the option which covers everything most accurate would be their true 12 bit oscilloscope for $800+, which no doubt would be great but seems overkill. Unless even fluctuations of far less than 1% are responsible for our eye strain, which at this point I have a hard time to believe.
The Hantek 6022BE has some accuracy problems in low voltage modes. Those modes are not real hardware modes but zoomed in from like the +/-100 or +/-200 mV mode. You will still see PWM, but maybe not tiny fluctuations (say below 5% voltage ripple) at low screen brightness. But thanks to the Android software HScope, the 6022BE might be the cheapest option to build a portable solution. We just need to make the photodiode probe circuit more robust so it can survive a trip in a bag easily. Then, with a powerbank and a compatible Android device, everything should be portable.