What can I do to lighten the brown strips that IPL left on my forehead??? (photos)

In addition to Dr. Emer's recommendations, another option is hydroquinone cream and can applied topically. Avoid sun, sunlamps, and tanning beds. Use sunscreen and wear clothing and eyewear that protects you from the sun. An in person evaluation with a dermatologist would be ideal.

This can be improved with a combination of lasers, Aerolase, microneedling/prp, skin lightening, peels, and skin care. Sun protection is essential. I recommend getting a formal evaluation with a cosmetic dermatologist. Best, Dr. Emer

Thanks for your question and posting pictures.  It is important for you and others with a similar should know to avoid.  IPL treatments are only safe on Skin Types I-III with no recent sun exposure.  Based on your skin type and working outside, you should have NEVER received an IPL treatment.  In addition, whoever performed the treatment apparently had very poor technique.  If you see stripes after your treatment, it wasn't overlapped properly.  Try the advice below to minimize the variations in your skin tone.  If that doesn't work, look for something like the PicoSure FOCUS.  It works very well on hyperpigmentation and is safe for all skin types.  Best of luck.

Do not do a chemical peel. You should make sure that you see the medical director - a doctor from the treating office so that they can know what happened and be involved in management. I know that it's frustrating, but this is called post inflammatory hyperpigmentation and is caused by inflammation. Any intervention which causes more inflammation in the first few months (like peels and more IPL) can make it worse. You should be placed on a cream containing hydroquinone and Retin-A for at least a few months with very, very strict sun avoidance and blocks. Anything left after at least 4-6 months of this can be cleaned up with a very skilled IPL practitioner or lasers.

It appears to be post inflammatory hyperpigmentation.  The best is to sunprotect with SPF of at least 30.  Use a photoprotector such as Vitamin C serum and at night utilize a retinoic acid (tretinoin, start with 0.05%) and add a hydroquinone (bleaching agent).  This will help to lighten.  It is important for all patients to not have a tan prior to IPL.  Some IPL's also have a melanin index reader where you can check the amount of melanin in the skin and adjust settings accordingly. 
I would first utilize the topicals prior to doing additional energy based modalities or peels.
Best of luck!