An apparatus with a combination of a point light source and a single lens is provided. The present apparatus includes a point light source, a photodetector and a lens. The lens is placed in the same side of the point light source and the photodetector in order that the light emitting from the point light source is focused onto a target area of an object through the lens. The reflected light from the target area is focused onto the photodetector through the lens. The present apparatus can qualitatively and quantitatively monitor a content of a specific component of a tested solution. The geometric relationship of the point light source, the photodetector and the single lens can improve a measuring resolution of the present apparatus.