The Batman Bat Signal would never actually work as portrayed in movies and comics due to how projectors and light sources behave.
A real working bat signal requires a convex lens to focus the light and create a projected image onto a screen.
Light sources, such as a spotlight, do not produce focused light in one uniform direction, making it unsuitable for a bat signal.
By using a lens, specifically a convex lens, light can be refracted to converge at a focal point and create a projected image.
To create a real working bat signal, powerful LED lights and a lens system were used to project the Batman logo onto a screen.