There are a lot of variables...

• Monitor calibration for gamma response/brightness
• Consistent ambient light levels- so monitor brightness remains consistent
• Actual scene luminance- not all histograms should be centered in the middle
• Color management employed by the program in use- not all will use the monitor's profile

LR and PS should be consistent for image brightness as they both use full color management and the monitor's profile. But Microsoft photo viewer does not; it is only partially color managed.

However, LR uses a variety of color spaces with different tone response curves (TRC)/gamma curves applied; and that will affect how an image appears. The working space in LR's develop module uses ProPhoto RGB with a linear TRC. The library module uses Adobe RGB (and previews are stored in Adobe RGB); this is to minimise step size with 8-bit previews. The web module uses sRGB. And Melissa RGB is used for histograms (to give a more perceptually uniform histogram).

If you open a raw file in PS/ACR you set the color space/TRC at the bottom of the ACR interface. If this is significantly different from the color space used in LR it will affect the colors and brightness notably differently. Also note that if you use different color backgrounds in LR/PS it will change the apparent brightness of the image you are viewing.

You cannot set LR's brightness separately from the monitor's brightness setting... nor any other program I know of. However you can use a gamma strip to verify the monitor's brightness is suitable for the ambient levels.

I use this image. When monitor brightness is appropriate you should just barely be able to see the difference between the alternating black squares in the top row; and you should be able to tell the difference between the brightest squares in the bottom row.

If you cannot set your brightness so that you can see all of the details of the test strip, then you need to calibrate your monitor; or it may just not be possible with your monitor.

The image is a png intended to be used as the identity plate in LR... if you wish to do so, be sure to save the file as a png. When added it will always be visible in the upper left corner.