One side is plated with a spectroscopic film and the other side is plated with an antireflection film, thus dividing a beam of light into a reflected beam of light and a transmitted beam of light. This film is formed by vacuum evaporation of multi-layer dielectric films, without the problem of metal film absorption, and the total ratio of transmittance to reflection reaches 100. According to the ratio of transmitted light to reflected light, there are mainly 10% anti-90%, 20% anti-80%, 30% anti-70%, 40% anti-60%, 50% anti-50% (semi-anti-semi-transparent), 60% anti-40%, 70% anti-30%, 80% anti-20%, etc.