Exposure Time Calculator

The Exposure Time Calculator helps determine how long a person, object, or film should be exposed to a certain source to achieve a desired effect. It is used in various fields, including radiation protection, medical imaging, and photography. Whether managing radiation doses for safety or fine-tuning lighting for the perfect photograph, this tool plays a crucial role in maintaining precision and protection.

By using this calculator, professionals can avoid overexposure in radiation environments and achieve the correct brightness and clarity in photographic setups. It serves as a practical decision-making aid across technical and creative fields.

Formula of Exposure Time Calculator

1. Radiation Dosimetry (Health Physics and Safety)

Exposure Time = Total Exposure / Exposure Rate

Where:

• Exposure Time is the duration of exposure (in seconds, minutes, or hours)
• Total Exposure is the target or recorded dose (e.g., in mR or Sv)
• Exposure Rate is how fast the exposure accumulates (e.g., in mR/h or Sv/h)

This version of the formula ensures safety in environments like radiology or nuclear energy, where minimizing risk is critical.

2. Photography (Camera Shutter Settings)

Exposure Time = Exposure Value / (Aperture² × Scene Luminance)

Or more commonly:

Exposure Time = 1 / Shutter Speed

Where:

• Exposure Time is the time the camera shutter stays open (in seconds)
• Shutter Speed is typically set as a fraction, like 1/60 or 1/125
• Aperture and ISO affect brightness, but shutter speed alone gives you the exposure time directly

This version helps photographers balance light sensitivity, depth of field, and motion blur.

Reference Table for Fast Lookup

Application	Input	Output (Exposure Time)
Radiation Safety	Exposure = 100 mR, Rate = 25 mR/h	4 hours
Camera Settings	Shutter Speed = 1/250	0.004 seconds (4 milliseconds)
Medical Imaging	Exposure = 2 mSv, Rate = 1 mSv/h	2 hours
Scene Lighting	Aperture = f/4, Luminance = 20	Calculated per exposure value

This table gives a quick idea of typical values before using a calculator for exact results.

Example of Exposure Time Calculator

Let’s say a worker is in a radiology area where the exposure rate is 50 mR/h. Safety guidelines allow no more than 200 mR of exposure. To find how long the worker can stay safely:

Exposure Time = 200 mR / 50 mR/h = 4 hours

So, they should limit their time in that zone to 4 hours.

In photography, if the shutter speed is set at 1/125, then:

Exposure Time = 1 / 125 = 0.008 seconds

That means the shutter remains open for 8 milliseconds, useful for capturing motion without blur.

Most Common FAQs