Image sensor noise was estimated in an approximately perceptually uniform space with a color image sensor model. Particularly, the noise level with respect to an image sensor's pixel pitch and the dark noise was investigated. It was shown that the noise level could be about half when spectral sensitivity was optimized considering noise with reduced color reproduction accuracy. It was also shown that for a 2.0µm pixel pitch sensor, the exposure index should be less than 100-150 in order to keep the noise level σ₉₄ less than 5 even if it had no dark noise, whereas the exposure index could reach about 2000-4000 for a 8.0µm pixel pitch sensor depending on the sensor sensitivity and the dark noise level.