I tried to capture the following small red light, but I failed. It's more white than red. How can I correctly capture the red light as seen through my eyes?
f-stop: f/5
exposure time: 1/30
ISO speed: 6400
Focal Length: 41mm
f-stop: f/4
exposure time: 1/40
ISO speed: 5000
Focal Length: 28mm
This is simply a problem of dynamic range. When the overall scene is evenly exposed (in this case, slightly underexposed), the light itself is too bright for the range of your sensor.
Assuming that you want both the light and the dial to be apparent in the scene (an assumption I make because you say you want to see it as you see through your eyes), you can take one of several different approaches:
Standard HDR techniques: take several images and blend them with programs meant to do this.
Try exposing the whole thing for the red light, and bringing up the rest using shadow correction. This will probably result in a lot of noise, but could work.
Simply taking two pictures, one exposed for the light and one exposed for the background, and manually masking the light and pasting it over the other.
You could also, as @ysap notes, light the surrounding area more brightly to bring the dial and background nearer to the brightness of the light. This might not give an effect that seems satisfying, though, as the red may seem more washed out than it does to your eye.
Or, if you just want to capture the light and don't really care about the rest, you can do as Philip suggests in the comment below and simply dial in negative EV compensation, to expose for the light alone.
If you make the surrounding brighter, you can decrease the scene's dynamic range. Try casting some light from a white light source on the device and make your exposure shorter.
You need to lower your exposure. The red is brighter than the camera can pick up. If it is bright enough, you may have to use an HDR technique and take one really dark photo for the red of the light and another for everything else.
The reason the red light appears white in the picture is that it is significantly overexposed. Even the small amounts of blue and green in the red light are enough to saturate your sensor in red and green, and almost saturate it in blue. Thus it appears almost white. Look at it this way, if your camera reaches saturation at a value of 255 and there is enough red in the light to reach 2500, enough green to reach 1000, and enough blue to reach 400 then all three channels will be totally saturated (255, 255, 255), even though there is more red than green or blue in the actual light.
To get the color of the red light to show accurately in your photo, you must reduce exposure until none of the color channels on your sensor are totally saturated by the light. Of course this will create problems with the rest of the scene which will be much darker. You will either need to illuminate the rest of the scene to raise it closer to the value of the red light or do as others have advised and combine two exposures: one properly exposed for the red light and another exposed for the rest of the scene.
