Heart rate (HR) is one of the most important vital signs for indicating the health condition of a person. Contactless camera-based HR measurement is particularly beneficial for sleep monitoring, as it is comfortable and convenient. However, compared with ambient light, the skin pulsation in near infrared range is much weaker and more susceptible to distortions (e.g. body motion, light changes). In this paper, we propose a method to expand the single-wavelength channel of a near infrared camera to multiple channels for illumination noise reduction, where the channel expansion is performed in the spatial domain using skin and non-skin pixels. The essence is using illumination changes of non-skin pixels to eliminate such a distortion on skin pixels and thus improve pulse extraction. On average, measurement coverage increased from 50% to 83% for the methods of subtraction and Segment Principal Component Analysis (Seg-PCA), and Signal-to-Noise Ratio (SNR) is increased from -8.40 dB to -4.62 dB for the method of Segment Independent Component Analysis (Seg-ICA).