Browse Definitions:
Definition

Contributor(s): Tom Higgins

The steradian (symbolized sr) is the Standard International (SI) unit of solid angular measure. There are 4 pi, or approximately 12.5664, steradians in a complete sphere.

A steradian is defined as conical in shape, as shown in the illustration. Point P represents the center of the sphere. The solid (conical) angle q, representing one steradian, is such that the area A of the subtended portion of the sphere is equal to r2, where r is the radius of the sphere.

A general sense of the steradian can be envisioned by considering a sphere whose radius is one meter (r = 1m). Imagine a cone with its apex P at the center of the sphere, and that intersects the surface in a circle (shown as a red ellipse, the upper half of which is dashed). Suppose the flare angle q of the cone is such that the area A of the spherical segment within the circle is equal to one meter squared (A = 1 m2). Then the flare angle of the cone is equal to 1 steradian (q = 1 sr). The total surface area of the sphere is, in this case, 12.5664 square meters (4 pi times the square of the radius).

Based on the foregoing example, the geometry of which is independent of scale, it can be said that a solid angle of 1 sr encompasses about 1/12.5664, or 7.9577 percent, of the space surrounding a point.

The number of steradians in a given solid angle can be determined by dividing the area on the surface of a sphere lying within the intersection of that solid angle with the surface of the sphere (when the focus of the solid angle is located at the center of the sphere) by the square of the radius of the sphere.

Also see radian and Standard International (SI) System of Units.

This was last updated in September 2005

#### Start the conversation

Send me notifications when other members comment.

## SearchCompliance

• ### risk map (risk heat map)

A risk map, also known as a risk heat map, is a data visualization tool for communicating specific risks an organization faces. A...

• ### internal audit (IA)

An internal audit (IA) is an organizational initiative to monitor and analyze its own business operations in order to determine ...

• ### pure risk (absolute risk)

Pure risk, also called absolute risk, is a category of threat that is beyond human control and has only one possible outcome if ...

## SearchCloudProvider

• ### cloud ecosystem

A cloud ecosystem is a complex system of interdependent components that all work together to enable cloud services.

• ### cloud services

Cloud services is an umbrella term that may refer to a variety of resources provided over the internet, or to professional ...

• ### uncloud (de-cloud)

The term uncloud describes the action or process of removing applications and data from a cloud computing platform.

## SearchSecurity

• ### federated identity management (FIM)

Federated identity management (FIM) is an arrangement that can be made among multiple enterprises to let subscribers use the same...

• ### cross-site scripting (XSS)

Cross-site scripting (XSS) is a type of injection security attack in which an attacker injects data, such as a malicious script, ...

• ### firewall

In computing, a firewall is software or firmware that enforces a set of rules about what data packets will be allowed to enter or...

## SearchHealthIT

• ### 21st Century Cures Act

The 21st Century Cures Act is a wide-ranging healthcare bill that funds medical research and development, medical device ...

• ### vendor neutral archive (VNA)

A vendor neutral archive (VNA) is a technology that stores medical images in a standard format and interface, making them ...

• ### HITECH (Health Information Technology for Economic and Clinical Health) Act of 2009

The HITECH (Health Information Technology for Economic and Clinical Health) Act of 2009 is legislation that was created to ...

## SearchDisasterRecovery

• ### crisis management plan (CMP)

A crisis management plan (CMP) is a document that outlines the processes an organization will use to respond to a critical ...

• ### business continuity and disaster recovery (BCDR)

Business continuity and disaster recovery (BCDR) are closely related practices that describe an organization's preparation for ...

• ### business continuity plan (BCP)

A business continuity plan (BCP) is a document that consists of the critical information an organization needs to continue ...

## SearchStorage

A bad block is an area of storage media that is no longer reliable for storing and retrieving data because it has been physically...

• ### all-flash array (AFA)

An all-flash array (AFA), also known as a solid-state storage disk system, is an external storage array that uses only flash ...

• ### volume manager

A volume manager is software within an operating system (OS) that controls capacity allocation for storage arrays.

## SearchSolidStateStorage

• ### hybrid hard disk drive (HDD)

A hybrid hard disk drive is an electromechanical spinning hard disk that contains some amount of NAND Flash memory.

Close