Photoshop layer which creates glow on edges

I'm making some amends on a poster for a client which was originally created by another designer. The original designer created what looks like a sort of 'glow' effect on just the edges of the images/elements in the poster by creating a layer as shown in the the attached and setting it to screen and 50% opacity. I can't for the life of me work out how they created this layer. To me it looks similar to a high pass layer but black and white but I tried this and I couldn't convert a high pass layer to black and white so I'm not sure if this is what they did. Any way if anyone has any ideas on how this layer is created I would be truely grateful. Thanks so much for your help!