# Calculate the number of photons at a pixel

We learn a great deal about image quality and noise limits by counting the (Poisson) arrival of photons at each pixel. Because ISET uses physical units throughout, we can easily calculate the number of incident photons, or stored electrons, at the sensor.

## Contents

```ieInit
```

## Load up an example scene

```sFile = fullfile(isetRootPath,'data','images','rgb','hats.jpg');
scene = sceneFromFile(sFile,'rgb', 100, displayCreate('OLED-Sony'));

```

## Show a region of interest on the scene

```% You can get the roiRect using get(gcf,'userdata')
roiRect = [64 64 16 16];
ieDrawShape(scene,'rectangle',roiRect);
```

## Plot the mean spectral radiance in the roi

```[udata, f] = scenePlot(scene,'radiance photons roi',roiRect);

% The sum of the mean number of photons from all the wavelengths
% q/s/sr/nm/m2
t = sprintf('Sum of photons across wavelengths %.2e\n',sum(udata.photons(:)));
title(t);
```

## Create spectral irradiance at the sensor for optics with a range of f#

```oi = oiCreate;  % Basic diffraction-limited optics

% Region in the OI we will measure
roiRect = [291 202 16 23];

% Loop for different f numbers
fnumbers = [2,4,8,16,32];

% Store the photon count here
totalQ = zeros(1,length(fnumbers));

for ff = 1:length(fnumbers)
oi = oiSet(oi,'optics fnumber',fnumbers(ff));
oi = oiCompute(scene,oi);
end
```

```vcNewGraphWin;
grid on
xlabel('Wavelength (nm)'); ylabel('Photons/s/nm/m^2');
```

## Plot the number of photons as a function of f#

```vcNewGraphWin;
loglog(fnumbers,totalQ,'-o');
grid on;
xlabel('f#'); ylabel('Photons/s/m^2')
title('Total photons vs. f#')
```