The phrase "alert the viewer" is correct and usable in written English.
It can be used when you want to notify or inform someone who is watching or observing something, typically in a visual context.
Example: "The documentary aims to alert the viewer to the urgent issues surrounding climate change."
Alternatives: "notify the viewer" or "inform the viewer".
Nearby is Édouard Manet's Young Woman Among Flowers (1879), where we painlessly accept the portrayal of an almost featureless woman standing among a morass of flowers that are impossible to classify: a smear of yellow, a smudge of carmine: all that is necessary to alert the viewer as to the content of the scene.
As I tuned in to watch the inaugural stateside broadcast, I expected a full HD presentation, flashing adverts, Ryan Seacrest vomiting nonsense into a microphone, and special US-only graphics packages flying in at random intervals to alert the viewer to a special episode of The Big Bang Theory where Sheldon open-mouth kisses a Klingon.
In a New York Times interview from November 2011, CBS executive Kelly Kahl said CBS was doing its best to alert the viewers.
There is no rhyme or reason as to why they use their mother tongue and having the mishmash of the two languages doesn't make it seem more authentic – it just alerts the viewer to the fact that they should be speaking in Spanish the whole time.
The unbound roundness of the work alerts the viewer that they're in a purely feminine domain.
The show at Arcature Fine Art combines the foreign and unusual while alerting the viewer to a sense of foreboding about what you are seeing.
But it was Tiger, and given that he has cameras on him at all times (and that, heading into the fifteenth Friday, he was atop the leaderboard, well-positioned for a weekend charge), a viewer called in to alert the rules committee that Tiger had taken an improper drop.
Viewers spotted the men in action and called to alert the park.
On "Top Chef," the Teleflora bouquets will be shown in the background, and a graphic will appear on the screen to alert viewers to the product, said Thomas Mooney, chief marketing officer for Teleflora.
The new rules will require broadcasters to alert viewers at the start and the end of shows which feature product placement, probably by using a symbol that appears briefly in the corner of screens.
MTV noted that the episodes were being shown only at or after 10 p.m. Eastern, and said in the statement, "We also have taken numerous steps to alert viewers to the strong subject matter so that they can choose for themselves whether it is appropriate".
