## Absorbance

The wavelength of the light can be in the range 320-800nm.

The intensity of the beam is measured without any sample (I0) first and then each well is measured (I). The absorbance is calculated from -

$\displaystyle{ A = -log(I/I_0) }$

Because the optical surfaces of the empty well reflect backward about 8% of optical energy, the measured absorbance values are always about 0.04 for an empty clear plate. On our machine, this background appears to be about 0.036.

## Absorbance at 600nm

To calculate cell density, we measure the absorbance of the culture at 600nm using the OD600 label.

### Linearity

Plate Reader and portable spec. measurements of culture density

To confirm that the absorbance reading is linear with respect to the culture density, the following experiment was performed. A stationary phase culture of BL21 in LB media was serially diluted in water by factors of 2. Plate reader and portable spec. measurements were made on all dilutions. The portable spec. was blanked with water and a water backgound was subtracted from all the plate reader measurements. All measurements were made in triplicate. The graph to the right shows that over the range 0.013 to 0.846, absorbance is linear with respect to concentration. Since the portable spec. saturates at an OD600 of 2, the plate reader can be used to measure a wider range of cell densities directly.