How to brighten image, maintaining contrast?

I've been trying out some of the lighten some images, is there a way to brighten an image, but maintain contrast .. so far I've played LIGHTEN, HARD_LIGHT and SOFT_LIGHT - but images end up looking washed out.. any idea of a better approach ?
 

S++



Code is pretty simple at the moment;


        LIGHTEN_ALPHA = 0.25
        LIGHTEN_OPERATOR = cairo.constants.OPERATOR_LIGHTEN

        width = source_surface.get_width()
        height = source_surface.get_height()


        lightened = cairo.ImageSurface( cairo.constants.FORMAT_ARGB32, width, height )
        with cairo.Context( lightened ) as cr:
            cr.set_source_surface(source_surface)

            cr.paint()

            cr.set_operator( LIGHTEN_OPERATOR )
            cr.set_source_rgba(1, 1, 1, LIGHTEN_ALPHA)
            cr.paint()

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