Lighten image through Script-fu

Please help. I lighten images using screen mode in GIMP but I want to do the same steps in batch but the following doesn’t produce the desired output.

(define (script-fu-lightn in out)
(let*
(
(image (car (gimp-file-load 1 in in)))
(drawable (car (gimp-image-get-active-layer image)))
(layer (car (gimp-layer-copy drawable 1)))
(gimp-layer-set-mode layer SCREEN-MODE)
(gimp-image-add-layer image layer -1)
)
(gimp-file-save 1 image layer out out)
)
)

I think that it is…
(gimp-layer-set-mode value-layer 5)

Thank you for that. it works now. i also needed to add a gimp-image-merge-down statement