# Optogenetics Power Calculator

#### Calculate Effective Stimulation Power

Irradiance drops rapidly in the brain, so how do you know what power you need from your optogenetics system? This calculator predicts the light power needed for effective stimulation for your study. Just input parameters according to your planned study and press Calculate. Scroll down for details and an explanation of effective stimulation power.

INPUTS
Type of brain matter:
• Grey matter eg. cortex
• Mixed eg. thalamus
• White matter eg. brainstem
• Depth (mm):
Fibre core diameter (um):
Numerical aperture (NA):

OUTPUT
Effective stimulation power (mW):

If you already know the power of your system and instead want to know the effective stimulation depth, use the depth calculator.

## How to use the power calculator

The parameters you need to input are:

1. Type of brain matter – the default option is a “mixed” intermediate density brain matter such as the thalamus. At the extremes are grey matter such as the cortex and white matter such as the brainstem.
2. Depth (mm) – the depth of brain tissue you want to have effective optogenetic stimulation. I recommend aiming for a minimum of 1 mm to maximise your chances given experimental variability.
3. Fibre core diameter (µm) – the diameter of your fibre, usually 200 µm for in vivo optogenetics. I recommend these from Thorlabs.
4. Numerical aperture (NA) – the numerical aperture of your implanted fibre. I recommend 0.22 NA fibres for lasers or LED’s.
5. Irradiance threshold (mW/mm^2) – the irradiance (ie. power per surface area) needed to activate your opsin of choice. 1 mW/mm^2 is a good value to start with, particularly for any ChR2-based opsins.

## Why use this power calculator?

The primary purpose behind this calculator is to inform you of your basic experimental needs when planning an optogenetics study. For example, say you were planning a new set of experiments and weren’t sure what equipment to buy. If you input your planned experimental parameters, this calculator estimates the light power you might need.

It can be difficult to know, for example, what kind of light source you will need to activate the opsin in your planned studies. And it generally doesn’t help going to the manufacturer’s websites, as they will always try to convince you their kit is best for your needs. You often find equipment for sale that is, frankly, not fit for purpose. However, it can be difficult to know that before you buy it.

## How it works

Light spreads and scatters out from the end of the optic fibre. This means that the further your neurones are from the end of the fibre, the more light power you need at the source in order to have sufficient irradiance to activate the opsin expressed in your neurones.

Here you can see predicted irradiance relative to distance from fibre tip with standard opto settings. The dotted horizontal line represents the irradiance required for effective stimulation of ChR2(h134r). The higher the light power, the further the light will travel before the irradiance drops below this threshold.

This power calculator simply tells you the light power needed to keep the irradiance above the stimulation threshold to the depth you have specified. This is called the effective stimulation power. Here you can see the massive increase in power needed to maintain effective stimulation with increasing distance from fibre tip.

## Numerical aperture is critical

The numerical aperture (NA) determines how much the light scatters when it leaves the fibre. The higher the NA, the more the light scatters and the greater power you need to activate neurones at the same distance from the fibre tip:

It might not be obvious which is the best NA fibre to use. For example, Plexon, Prixmatix and Doric all recommend 0.66 NA fibres for their LED optogenetics systems. To investigate the effect of NA, I calculated light power needed for effective stimulation of ChR2 to a depth of 1 mm. Here you can see the massive increase in power needed with increasing NA to achieve effective stimulation. Note that the only parameter that changes here is the NA. Increasing NA makes your system far less effective.

## A practical example

Say you wanted to set up a new optogenetics system for eNpHR3.0 activation in the 550-600 nm range. Validation by e-phys might show a threshold irradiance of 3 mW/mm^2. Inputting this into the power calculator with a stimulation depth of 1 mm would give an estimated power requirement of 13.8 mW. That would likely require a laser to achieve sufficient power.

If we wanted to use a Plexon system to stimulate eNpHR3.0, the best option would be their 550 nm LED. Plexon claim that LED produces 8.8 mW from a 0.66 NA fibre under optimal test conditions. This might seem close enough to the 13.8 mW figure to be usable.

However, if we input those values into the depth calculator, we find an effective stimulation depth of just 0.40 mm. This very short stimulation distance would theoretically be usable, but only for a very small target. However, experimental variability would inevitably result in a high experimental failure rate.

In fact, in order to achieve just 1 mm effective stimulation with their system as recommended, you would need 84.9 mW. This is almost a 10-fold increase over its maximum output. The impact of NA on light scatter like this can be deceptive. So you can’t just match power output from someone else’s study using a different NA fibre – we can see it doesn’t work that way.

## Further info

I will reiterate what I have said in my blog posts: this optogenetics power calculator provides a predicted estimate of the required light power according to the parameters you have input. So, please don’t take it as an absolute truth, but it should be helpful as a guide.

For further info about development of the calculator tool, please check out this blog post. To summarise the science, it is based on calculations by Aravanis et al.1 and uses scattering coefficients from Aravanis et al. and Yaroslavsky et al.1,2:

• Grey matter: 11.2
• Thalamus (intermediate scattering): 20
• White matter: 40

If you use this calculator to inform your experiments, please acknowledge it as: https://nicneuro.net/optogenetics-power-calculator

1. Aravanis et al. J Neural Eng 4, S143-S156 (2007) An optical neural interface: in vivo control of rodent motor cortex with integrated fiberoptic and optogenetic technology
2. Yaroslavsky et al. Phys Med Biol 47, 2059 (2002) Optical properties of selected native and coagulated human brain tissues in vitro in the visible and near infrared spectral range