Photoshop: How do I create a Luminosity mask?

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Hi.
I am a psychology student and I have a photograph of sleep related items and neutral items in the same photograph. Now I need to make sure that the luminosity is even across the photograph with no attention grabbing bright areas. Does any one know how to reduce overly bright areas of the photograph using a luminosity mask. Also I need some kind of measurement of luminosity across the photo, basically to show which areas have higher luminosity.
Thanks
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Richard Hildred-Saliba

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Posted 2 years ago

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Cristen Gillespie

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Unless you want to buy an inexpensive extension, such as TK Actions (there are a few others out there, as well), or use web sources to learn to create your own luminosity masks, you can  use Select > Color Range > Highlights, and set the slider to include just those that are above a given level of luminance. Maybe you want to affect anything brighter than 235 (RGB with a scale of 0-255). Set the Range slider to 235 and the set the Fuzzy slider to build in some feathering.

Once you've got the mask that reveals the bright areas you need to dim, you can use Levels, Curves, B/C to darken those areas, with the mask to restrict the adjustment layer to the light portion of the mask. You probably also want to set the adjustment layer to Luminosity blending mode to avoid problems with color. Of course, if you know what areas need darkening, you can also hand paint a mask, but that's a bit more time consuming. You may have to do some hand painting anyway.

> Also I need some kind of measurement of luminosity across the photo, basically to show which areas have higher luminosity.>

An easy way, especially if you're using Color Range, is to create a Curves layer, run your eyedropper over the image, and read the input field as you move about. Numbers will indicate a luminance between 0-255 for the RGB composite channel. The Curves adjustment layer input helps figure out what number you should use for Range in the Color Range dialog. You can scan the image, noting the inputs over various locations, until you're satisfied that you've got the lowest number you need to catch the brightest areas.
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Mark Heaps, Champion

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So you can hold your CMD key on a Mac or CTRL on a Windows machine, and click on the RGB channel. This will load a selection of the bright spectrum of the luminosity range within the image. Once you have that selected you have a LOT of options on how to reduce the brightness in those areas.

You can use an Image Adjustment like Exposure, and reduce the exposure in those areas, or you can use Levels and grab the output controller on the right for the brightest output value and bring that in to reduce overall contrast and brightness at the top end.

For what it's worth, when I have punchy highlights I use Selective Color. It's a bit abstract but gets great results. Choose Selective Color, and set the pull down menu (says REDS by default) to "Whites". Now go down to the Blacks slider while in the range and push the blacks value up. This will decrease the high key spectrum of your image more subtly reducing the highlights. There's a screenshot below.

Good luck!
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Cristen Gillespie

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The limitation with using the default luminosity mask created by cmd/ctrl-clicking on the RGB channel thumbnail is you're loading everything that is 50% light and above. If you want to restrict the selection to just those luminances which are too bright, you'll need to find a way to restrict that selection even more.

I've used the Selective Color dialog to add some black to whites on occasion. That might be a good way to get at whites without using a luminosity mask. It's not going to select as much as loading the default luminosity mask, that's for sure. It's much more restrictive.

 You might need to try the Absolute radio button to bring the luminance down enough for these tests the OP is attempting, and anything that injects a lot of black into a color is going to look pretty bad at higher settings. Something to play around with. . .
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David Converse

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A selection is the same as a mask, which is the same as an 8-bit greyscale image. Using the default channel select icon and adjusting that mask with Curves, Levels, etc works quite well.
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Craig Cartmell

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