> For the complete documentation index, see [llms.txt](https://khalil-lab.gitbook.io/evolver/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://khalil-lab.gitbook.io/evolver/hardware/smart-sleeve/optical-density/od90-vs-od135.md).

# OD90 vs OD135

## Definition

The two photodiodes on the eVOLVER are at 90 degrees and 135 degrees from the IR LED.

## Recommendations

* Try OD135 for OD600 < 0.5
  1. There will likely be vials that max out between OD600 0.5 and 1.0
* Try OD90 for OD600 > 0.5
  1. Some eVOLVER systems are able to read OD90 up to OD600 of 4 or 5
  2. This may require optimization of the internal resistor pack
* Switching between currently requires [manual OD calibration](/evolver/getting-started/calibrations/manual-calibration-calibrate.py.md) (can be applied to an existing GUI calibration you have on your eVOLVER)
* See [this ](https://www.evolver.bio/t/od-calibration-issues-signal-maxed-out/447)forum post for an example