To measure the brightness of a light bulb, you would use a light meter. A light meter is a device that measures the intensity of light. It is typically used to measure the amount of light falling on a surface or the amount of light emitted by a source. Light meters are often used in photography, videography, and other fields where precise lighting is essential.

There are two main types of light meters: spot meters and incident meters. Spot meters measure the light falling on a specific spot, while incident meters measure the light falling on the meter itself. Spot meters are typically used to measure the light falling on a subject, while incident meters are typically used to measure the light falling on a camera lens.

Light meters can be either analog or digital. Analog light meters typically use a needle to indicate the light intensity, while digital light meters typically use a digital display. Digital light meters are often more accurate and easier to use than analog light meters.

When using a light meter, it is important to hold the meter at the same distance from the light source as the subject or camera lens. This will ensure that the light meter is accurately measuring the light intensity.