Signal-to-Noise Ratio Comparison of CardioPDA and CardioCCD

In terms of signal-to-noise ratio the best a camera system can do is to approach the shot-noise limit. Some of the factors in the camera design affect how close the camera approaches shot-noise performance and others affect the range of light intensities over which this performance is achieved. In addition, extraneous factors can make the performance worse than the shot-noise limit.

Shot Noise

The limit of accuracy with which light can be measured is set by the shot noise arising from the statistical nature of photon emission and detection. Fluctuations in the number of photons emitted per unit time will occur, and, if an ideal light source emits an average of N photons/ms, the root- mean-square deviation in the number emitted is the square root of N. The effects of this relationship are indicated by the green line in the Figure which plots the light intensity divided by the noise versus the number of photons measured per ms. At high intensities this ratio is large and thus small changes in intensity can be detected. For example, at 10¹⁰ photons/ms a fractional intensity change of 0.1% can be measured with a signal-to-noise ratio of 100. On the other hand, at low intensities this ratio of intensity divided by noise is small and only large signals can be detected. For example, at 10⁴ photons/msec the same fractional change of 0.1% can be measured with a signal-to-noise ratio of 1 only after averaging 100 trials.

The figure also indicates the performance of the two RedShirtImaging systems, the photodiode array system (CardioPDA; blue line ) and the CCD camera system (CardioCCD; red line). CardioPDA approaches the shot-noise limitation over the range of intensities from 3x10⁶ to 10¹⁰ photons/ms. This is the range of intensities obtained in absorption measurements and fluorescence measurements on intact hearts. On the other hand, CardioCCD approaches the shot noise limit over the range of intensities from 5x10³ to 5x10⁶ photons/ms. This is the range of intensities obtained from fluorescence experiments on individual cells.

Saturation

The high intensity limit of the CardioCCD is set by the light intensity which fills the electron wells on the CCD chip. This accounts for the bending over of the camera performance at segment B in the Figure. Even though the CardioCCD camera has a large well-size compared to other CCD cameras, it will not be optimal for measurements of absorption or fluorescence measurements on in vitro slices and intact brains. The light intensity would have to be reduced with a consequent decrease in signal-to-noise ratio.

The CardioCCD saturates at light levels that result in resting light intensity readings of 0.3 volts at 1 Gohm on CardioPDA. In fact, the CardioCCD saturates at about the light intensity where the CardioPDA becomes shot-noise limited. Thus, as indicated in the figure, the range of light intensities where the two systems are optimal have very little overlap.

Dark Noise

Dark noise will degrade the signal-to-noise ratio at low light levels. Because the CardioCCD is a cooled CCD camera, its dark noise is much lower than that of the CardioPDA. The excess dark noise in CardioPDA accounts for the fact that segment C (for CardioPDA) in the Figure is substantially to the right of segment D (for the CardioCCD). The dark noise of the CardioCCD is lower than other commercially available CCD's in its performance range.

Extraneous Noise

A second type of noise, termed extraneous or technical noise, is apparent at higher light intensities where the sensitivity of the measurement is high because the fractional shot noise is low. There are several sources of extraneous noise. One type is caused by fluctuations in the output of the light source. Other sources are vibrations and movement of the preparation. Extraneous noise accounts for the break in the CardioPDA curve at A.