Higher luminosity usually means high power consumption (screens need the most energy in a laptop). You may need to depend on a power outlet rather than a battery. Bring an extension cord or a backup battery pack. On a Macbook, increase brightness with F2 and decrease with F1. PC laptops typically have brightness controls on the top row of keyboard keys, indicated by an icon of the sun and a plus (+) sign to increase, minus (-) to decrease. Depending on your system, you may need to hold down the Fn key as you press the buttons. Press Win+S to launch the search bar, then type "ease" into the box. When Ease of Access Center appears in the search results, click it to you're your accessibility options. Click "Make computer easier to see." Under "High Contrast," click "Choose a High Contrast Theme," then click one of the four options with a black background. In the Apple menu, select "System Preferences," then "Universal access." Click on the "Seeing" tab and scroll down to "Display." Right now, "Black on White" is checked. Place the check in "White on Black" instead. Slide the "Enhance contrast" slider to the right to increase the contrast between dark and light colors. A higher contrast will help you view the screen in sunlight. You can toggle quickly between "Black on White" and "White on Black" mode by pressing Control+ Option+ Command+8. Many different companies offer hoods or screens that you place over your screen to decrease sun glare. Check out options like the CompuShade SunHood and NuShield DayVue. Visit your local camping or hiking store and look for laptop sunscreens. These handy items will be more expensive than buying a little box, but they provide the added bonus of protecting your computer from the elements. Stores like Target and Ikea sell black square storage boxes/cubes that can be used with certain types of shelves. These boxes also make great DIY laptop hoodssimply slide your laptop inside (with the open side facing you) and use your laptop as normal. You can also try using a regular cardboard box.

Summary:
Hike up the luminosity (brightness) in the display settings. Turn on High Contrast Mode in Windows. Boost the contrast on your Macbook. Buy a laptop hood or sun-screen. Build your own laptop hood.