View White Paper by Interactive Advertising Bureau (IAB Spain)
In this research the IAB and OJD recommend audience measurement with wide angle devices - A solution provided by TruMedia

Actual Impressions - The number of people who look at your displays 
Length of Impressions - For how long people look at your display
Potential Audience Size -Number of people walking by
Dwell Time - How long people stay near your displays
Anonymous Demographics - Demographics of your audience (gender and age-group)

The crucial measurement information above, enables you to understand how well does your media performs.
Utilizing these metrics help you have additional benefits which includes:
Understanding ROI
Understanding ROO (Return on Objectives) such as changes in consumer behavior
Determining the best locations for displays
Tailoring screen content based on audience characteristics
Providing proof of performance metrics for your displays
Understanding audience engagement levels
Comparing metrics with industry standards including CPM, CPP, and GRP
Offering pay-per-impression billing for achieved audience targets
Optimizing advertising based on accurate audience measurement data

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