What are raccoon eyes? They are the dark areas in a model’s eye sockets caused by overhead lighting or mid-day sun.
There are multiple ways to prevent raccoon eyes in your photos.
One way is to pay attention to the direction of your main light. Make sure to position the model so that the light does not come in from above their head which will cast a shadow on their eyes from their brow. This is not the best method on a sunny day because the model will tend to squint and their eyes will be nearly closed in the photos.
A better method is to use a flash or a reflector to add light to their eyes. This can be done inside or outside. This also gives their eyes a little pop and a sparkle (catch light). The model can also face away from the sun to allow them to open their eyes.
I typically shoot in the shade or place the model with the sun to their back. I use a flash in a soft box for almost all photography, because I love the pop it gives and the sparkle in their eyes. A reflector will have the same effect if there is available light to reflect. I use a flash because its much faster to set up, I don't need an assistant to hold it, and I don't need to worry about the direction of available light.
The 2 photos above show how adding flash effects the photo. These were both shot in the shade, but there is still a notable improvement in the right photo with a flash. Not only are the shadows under her brows reduced, but the shadows in other areas are reduced as well. Also using flash allows you to expose for the background and then light the model separately with flash. You can see that the sky is blue and has more detail when exposing for the background and using flash on the model.