I believe I have a solution that will work though not as elegant as I desire (there may be other ways yet this was the first that came to min)
I will assume that you all values of your image are R=G=B for all pixels if in RGB or you are in grayscale mode.
1) First you need to know for color space you are in (e.g. sRGB, Adobe RGB, ProPhoto RGB, what the RGB values are for the desired range of L that you want to capture in a selection. E.G for sRGB L=40 is when R=G=B= 40. Also for L=60 R=G=B = 144 (or 145). You can determine these values by going into the color checker, set the L value and observe the RGB values
2) Above you B&W Layer/Image create a simple all White Layer
3) Set the Blend If sliders to grayscale and set the left slider to 144 and the right slider to 94 (NOTE - the sliders will cross pass each other to make this happen). What you should now see as in the example below is all areas that are not between L=40 and L=60 to be white.
4) Select the white area with the magic wand with tolerance set to 0 and no anti alias (unless that is what you want). This is the inverse selection of what you want.
5) Just do Enter+Cmd+I (Shift+Cntrl+I on PCs) to invert the selection and you have now selected the desired L range
Not sure this approach is perfect yet seems to do the trick. There could be corner cases where it does not work as I have not tried all possible cases. Here are the images
First image is a grayscale gradient just as an example with the Layer Styles shown yet not activated (Layer was turned off)
[ATTACH=full]97756[/ATTACH]
Here is the same shot with the White Layer visible and Layer Style Active
[ATTACH=full]97757[/ATTACH]
Here is selecting the white areas:
[ATTACH=full]97758[/ATTACH]
And here it is with the selection inverted
[ATTACH=full]97759[/ATTACH]
Hope this works for you
John Wheeler
PS - The numbers on the images won't match the numbers I gave you because they were screen shots which are in my monitors colorspace.
PPS - The above technique does not work if the selection range includes L=0 or L=100. That can be solved with another approach using the same numbering scheme to translate L to RGB and using a Threshold Adjustment Layer instead of a white Layer with the Layer Style FYI
